`!plugin load` should issue feedback on failure

[MostAwesomeDude]

While I recognize that this could potentially be very spammy, it should be possible for the administrator to get feedback on why a plugin could not be loaded.

[justinnoah]

@MostAwesomeDude This may be due to my lack of not knowing Twisted's plugin system, however, are registerPlugin (hamper.plugins.plugin_utils#57) and removePlugin (hamper.plugins.plugins_utils#76) missing methods on PluginLoader?