GUI: preparation for custom error handling

 - Standard error widget can now be generated by method in UiElements.
 - Call URL and posted object can now be injected to PerunError and by this
   passed to custom handling in method onError(PerunError error) of callback.
- Modified JsonClient and JsonPostClient to reflect this error.
- Source formatting needed because of formatting change in sources.
GUI: custom error handling for AddMember call

 - Custom error handling for AddMember callback for
   AlreadyMemberException, WrongAttributeValueException
   and WrongReferenceAttributeValueException.
 - Fixed mistaken error.getType() for error.getName().
GUI: custom error handling for more calls

- Added custom error handling for calls AssignGroupToResources
  and AssignGroupsToResource.
- Fixed custom error texts.
Merge pull request #233 from zlamalp/exception

GUI: custom exception handling