Add exceptions to git branch.
cleanup exceptions in checkout.
Refactor git/exception.dart.
Better exception throwing in git fetch.
Better exceptions in index.dart.
more exception handling.
Add exceptions in objectstore.
comments.
Merge branch 'master' into exception

Conflicts:
	ide/app/lib/git/exception.dart
Merge pull request #2234 from dart-lang/better_exception

Better Git exceptions