Allow to check if a request value has an error of a certain kind

[mikey]

Currently it is only possible to check if a request value got an error while filtering. To check for a specific error one must retrieve all errors from the request instance and loop through them. A better way would be a new method in the request class: public function hasValueErrorWithId($valueName, $errorId, $source = stubRequest::SOURCE_PARAM);

[mikey]

Implemented with changeset 1021.