* Extract common stuff into the base class ClangException
* Remove unused exceptions TranslationUnitParseErrorException and
  TranslationUnitReparseErrorException
* Do not send error messages to the Qt Creator side. The messages were
  only generated when the backend crashed and while it was not yet fully
  re-initialized (e.g. do code completion right after crash where the
  document was not yet registered at the backend).

Change-Id: I91d98d5ef681ad487f7a2fd66f78fa7cd1e958df
Reviewed-by: David Schulz <david.schulz@qt.io>