Rename exception names.
Add SparkErrorMessages class.
Don't show raw exception messages.
Improvements in spark exception class.
Catch action exceptions at top level.
throw the missid unthrown exceptions.
bug fixes.
Comments.
Merge branch 'master' into better_exceptions

Conflicts:
	ide/app/lib/exception.dart
	ide/app/lib/scm.dart
	ide/app/spark.dart
Merge pull request #2417 from dart-lang/improve_exception_handling

Improve exception handling