improve notification and error handling for review & submit action

- show message in status bar when these actions are successfully executed
- some refactoring for more intuitive exception handling in GerritUtil
  (make sure consumers/callbacks are only executed, when REST call
  was successful or no error message was provided)
- resolves #90
Merge pull request #95 from forrert/features/actions-notifications

improve notification and error handling for review & submit action