Improve error reporting.

Improve error reporting.

In class/Microsoft.Build.Engine/Microsoft.Build.BuildEngine:
	* Target.cs (TargetFile): New. Gives the name of the file
	containing the target definition.
	* BuildEngine.cs (.ctor): Add a 'taskfile' parameters, and
	return this for ProjectFileOfTaskNode.
	* BuildTask.cs (LogTaskStarted):
	(LogTaskFinished): Use parentTarget.ProjectFileOfTaskNode for
	the event.
	(InitializeTask): Set BuildEngine's task file to parent target's
	TargetFile.
	* Engine.cs (LogProjectStarted): Incase of default targets, set
	TargetNames to String.Empty .
	* TargetBatchingImpl.cs (LogTargetStarted):
	(LogTargetFinished): Use target.TargetFile for the event.

	* ConsoleLogger.cs: Keep a stack of project/target/task being
	executed. And corresponding list of errors and warnings.
	(EventsToString): Stringifies the current "stack", to be used
	as key to the table of errors/warnings and for show messages
	at the end of the build.

In class/Microsoft.Build.Tasks/Microsoft.Build.Tasks:
	* ResolveAssemblyReference.cs: Remove unnecessary tab in the warning
	messages.

In class/Microsoft.Build.Utilities/Microsoft.Build.Utilities:
	* TaskLoggingHelper.cs: Use the buildEngine.ProjectFileOfTaskNode
	in the event args.

git-svn-id: svn+ssh://mono-cvs.ximian.com/source/trunk/mcs@143937 e3ebcda4-bce8-0310-ba0a-eca2169e7518