Bug 1010: Implement restconf error responses

- RestconfError (new): encapsulates error information as defined in the
  RESTCON RFC.

- RestconfDocumentedException (new): exception that wraps RestconError info.

- RestconfDocumentedExceptionMapper (new): JAX-RS ExceptionMapper that
  translates a RestconfDocumentedException appropriately to XML or
  JSON depending on the user's mime type.

- JsonMapper: modified to handle null child schema as restconf error-info
  is defined as 'anyxml' and thus no schema may be present.

- ControllerContext: added getRestconfModuleErrorsSchemaNode method to
  obtain the 'errors' container schema from the ietf-restconf module.

- Removed ResponseException and changed occurrences in various classes that
  threw ResponseException to throw RestconfDocumentedException instead.

- Added unit tests for new classes and modified existing ones
  accordingly.

Change-Id: Idbe6f6cae9b40ba14701ee05dfadfdd51e961c6b
Signed-off-by: tpantelis <tpanteli@brocade.com>