Merge pull request #1090 from bentayloruk/1082-print-cmd-line-warning

WIP Warn when fail to parse cmd line args. Close #1082

src/app/FAKE/Program.fs

@@ -126,6 +126,12 @@ try
 
         //None of the new style args parsed, so revert to the old skool.
         | Choice2Of2(ex) ->
+
+            // #1082 print a warning as we've been invoked with invalid OR old-style args.
+            traceImportant "Error parsing command line arguments.  You have a mistake in your args, or are using the pre-2.1.8 argument style:"
+            exceptionAndInnersToString ex |> traceImportant
+            trace "Attempting to run with pre-version 2.18 argument style, for backwards compat."
+
             if (cmdArgs.Length = 2 && paramIsHelp cmdArgs.[1]) || (cmdArgs.Length = 1 && List.length buildScripts = 0) then printUsage () else
             match Boot.ParseCommandLine(cmdArgs) with
             | None ->

src/app/FakeLib/TraceHelper.fs

@@ -52,8 +52,9 @@ let traceFAKE fmt = Printf.ksprintf (fun text -> postMessage (ImportantMessage t
 let traceError error = postMessage (ErrorMessage error)
 
 open Microsoft.FSharp.Core.Printf
-/// Traces an exception details (in red)
-let traceException (ex:Exception) =
+
+/// Converts an exception and its inner exceptions to a nice string.
+let exceptionAndInnersToString (ex:Exception) =
     let sb = Text.StringBuilder()
     let delimeter = String.replicate 50 "*"
     let nl = Environment.NewLine
@@ -85,7 +86,10 @@ let traceException (ex:Exception) =
             if (e.InnerException <> null)
             then printException e.InnerException (count+1)
     printException ex 1
-    sb.ToString() |> traceError
+    sb.ToString()
+
+/// Traces an exception details (in red)
+let traceException (ex:Exception) = exceptionAndInnersToString ex |> traceError
 
 /// Traces the EnvironmentVariables
 let TraceEnvironmentVariables() =