plugin_register(): improve invalid plugins handling

Previously, the function would return null if the plugin could not be
registered.

It now exclusively returns a MantisPlugin object, and in case of
registration errors, the returned object will be a special subclass:

• InvalidPlugin: if the plugin's name or version is not defined
• MissingClassPlugin: if the source code can't be found or loaded

These errors will also be added to the MantisBT log (LOG_PLUGIN event).