Improve error message in case resolve fails

Currently Tycho gives a rather vague message ('see log for details') if
a resolve fails for a project what requires the user to search and match
the actual error if the resolving of a project fails.

This now adds the errors directly to the exception what gives a much
better info to the user in case of resolve failure.

tycho-core/src/main/java/org/eclipse/tycho/core/maven/TychoProjectExecutionListener.java

@@ -39,12 +39,14 @@
 import org.eclipse.tycho.ArtifactDescriptor;
 import org.eclipse.tycho.ArtifactKey;
 import org.eclipse.tycho.DependencyArtifacts;
+import org.eclipse.tycho.DependencyResolutionException;
 import org.eclipse.tycho.ReactorProject;
 import org.eclipse.tycho.core.TargetPlatformConfiguration;
 import org.eclipse.tycho.core.TychoProject;
 import org.eclipse.tycho.core.TychoProjectManager;
 import org.eclipse.tycho.core.osgitools.DefaultArtifactDescriptor;
 import org.eclipse.tycho.core.osgitools.DefaultReactorProject;
+import org.eclipse.tycho.p2.resolver.ResolverException;
 import org.eclipse.tycho.p2maven.DependencyChain;
 import org.eclipse.tycho.p2maven.InstallableUnitGenerator;
 import org.eclipse.tycho.resolver.TychoResolver;
@@ -105,7 +107,22 @@ public void beforeProjectLifecycleExecution(ProjectExecutionEvent event) throws
         try {
             legacySupport.setSession(mavenSession);
             //FIXME should return tycho project!
-            resolver.resolveProject(mavenSession, mavenProject);
+            try {
+                resolver.resolveProject(mavenSession, mavenProject);
+            } catch (DependencyResolutionException e) {
+                ResolverException resolverException = findResolverException(e);
+                if (resolverException == null) {
+                    throw new LifecycleExecutionException(
+                            "Cannot resolve dependencies of project " + mavenProject.getId(), null, mavenProject, e);
+                } else {
+                    throw new LifecycleExecutionException(
+                            "Cannot resolve dependencies of project " + mavenProject.getId() + System.lineSeparator()
+                                    + " with context " + resolverException.getSelectionContext()
+                                    + System.lineSeparator() + resolverException.explanations()
+                                            .map(exp -> "  " + exp.toString()).collect(Collectors.joining("\n")),
+                            null, mavenProject);
+                }
+            }
             TychoProject tychoProject = projectManager.getTychoProject(mavenProject).orElse(null);
             if (tychoProject != null) {
                 try {
@@ -131,6 +148,21 @@ public void beforeProjectLifecycleExecution(ProjectExecutionEvent event) throws
         }
     }
 
+    private ResolverException findResolverException(Throwable t) {
+        if (t != null) {
+            if (t instanceof ResolverException re) {
+                return re;
+            }
+            for (Throwable sup : t.getSuppressed()) {
+                if (sup instanceof ResolverException re) {
+                    return re;
+                }
+            }
+            return findResolverException(t.getCause());
+        }
+        return null;
+    }
+
     private Set<MavenProject> checkBuildState(TychoProject tychoProject, MavenProject project) throws CoreException {
         ReactorProject reactorProject = DefaultReactorProject.adapt(project);
         DependencyArtifacts artifacts = tychoProject.getDependencyArtifacts(reactorProject);

tycho-core/src/main/java/org/eclipse/tycho/p2/resolver/ResolverException.java

@@ -13,7 +13,12 @@
  *******************************************************************************/
 package org.eclipse.tycho.p2.resolver;
 
+import java.util.Collection;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
 import org.eclipse.core.runtime.IStatus;
+import org.eclipse.equinox.internal.p2.director.Explanation;
 import org.eclipse.tycho.core.shared.StatusTool;
 
 /**
@@ -25,13 +30,19 @@ public class ResolverException extends Exception {
     private static final long serialVersionUID = 1L;
     private final String details;
     private final String selectionContext;
+    private Collection<Explanation> explanation;
 
     public ResolverException(String msg, Throwable cause) {
         super(msg, cause);
         selectionContext = "N/A";
         details = "N/A";
     }
 
+    public ResolverException(Collection<Explanation> explanation, String selectionContext, Throwable cause) {
+        this(explanation.stream().map(Object::toString).collect(Collectors.joining("\n")), selectionContext, cause);
+        this.explanation = explanation;
+    }
+
     public ResolverException(String details, String selectionContext, Throwable cause) {
         super("See log for details", cause);
         this.details = details;
@@ -52,4 +63,8 @@ public String getSelectionContext() {
         return selectionContext;
     }
 
+    public Stream<Explanation> explanations() {
+        return explanation.stream();
+    }
+
 }

tycho-core/src/main/java/org/eclipse/tycho/p2resolver/ProjectorResolutionStrategy.java

@@ -91,8 +91,7 @@ public Collection<IInstallableUnit> resolve(Map<String, String> properties, IPro
             // log all transitive requirements which cannot be satisfied; this doesn't print the dependency chain from the seed to the units with missing requirements, so this is less useful than the "explanation"
             logger.debug(StatusTool.toLogMessage(s));
             explainProblems(explanation, MavenLogger::error);
-            throw new ResolverException(explanation.stream().map(Object::toString).collect(Collectors.joining("\n")),
-                    selectionContext.toString(), StatusTool.findException(s));
+            throw new ResolverException(explanation, selectionContext.toString(), StatusTool.findException(s));
         }
         if (s.getSeverity() == IStatus.WARNING) {
             logger.warn(StatusTool.toLogMessage(s));