Fix error detection in plugin helper

Looking through the plugin helper service just now (to shamelessly steal it for the Autocomplete module), I realized that, when moving it from the index to a service, we didn't adapt the error detection to the new mechanism. While we now retrieve the plugin manager just from the service class' properties, we still check for a ServiceNotFoundException to see whether $type is known.
No real problem there, since those methods are protected and thus only called with the (valid) $type parameters listed in the class itself, but still, sloppy coding.