Fixing minor validation errors

- Fixing description rule to allow references to define a description
- Fixing AutoRest logging output to avoid throwing NRE when an error is logged without an exception
- Changing the message that gets output for validation errors to make it more useful

src/core/AutoRest.Core/AutoRest.cs

@@ -57,7 +57,7 @@ public static void Generate(Settings settings)
 
                 if (messages.Any(entry => entry.Severity >= settings.ValidationLevel))
                 {
-                    throw ErrorManager.CreateError(Resources.CodeGenerationError);
+                    throw ErrorManager.CreateError(null, Resources.ErrorGeneratingClientModel, "Errors found during Swagger validation");
                 }
             }
             catch (Exception exception)

src/core/AutoRest.Core/Logging/ErrorManager.cs

@@ -30,7 +30,7 @@ public static CodeGenerationException CreateError(Exception exception, string me
             }
 
             var errors =
-                Logger.Entries.Where(e => e.Severity == LogEntrySeverity.Error).Select(e => e.Exception).ToList();
+                Logger.Entries.Where(e => e.Severity == LogEntrySeverity.Error).Select(e => e.Exception).Where(e => e != null).ToList();
             Logger.Entries.Add(new LogEntry(LogEntrySeverity.Fatal, FormatMessageString(message, args))
             {
                 Exception = exception

src/modeler/AutoRest.Swagger/Validation/DescriptionRequired.cs

@@ -14,7 +14,7 @@ public class DescriptionRequired : TypedRule<SwaggerObject>
         /// <param name="entity"></param>
         /// <returns></returns>
         public override bool IsValid(SwaggerObject entity)
-            => entity == null || entity.Description != null || string.IsNullOrEmpty(entity.Reference);
+            => entity == null || entity.Description != null || !string.IsNullOrEmpty(entity.Reference);
 
         public override ValidationExceptionName Exception => ValidationExceptionName.DescriptionRequired;
     }