No reason to have separate _getListOrObject-method in Gitlab-class.

Changed GitlabObject-class version of _getListOrObject to classmethod.
Removed _getListOrObject-method from Gitlab-class.
Changed Gitlab-class to use GitlabObject-class version of _getListOrObject
Raise NotImplementedError on all cases, where can*-boolean is False
Improved exception-classes.

- Added http status-code and gitlab error message to GitlabError-class.
- Added new GitlabOperationError-class to help separate connection and
authentication errors from other gitlab errors.
Improved error reporting

- Try to parse error response from gitlab.
- Use one function (_raiseErrorFromResponse) to parse and raise exceptions
  related to errors reported by gitlab.