[GUSDEV] non-obvious error

[Robert O. <ol...@mc...>]

FYI from a new user:

If you forget ga +meta --commit and try to register a plugin, you get  
confusing errors like this:

$ ga +create GUS::PluginMgr::GusApplication --commit

USER ERROR: GUS::PluginMgr::GusApplication has never been registered.
Please use 'ga +create GUS::PluginMgr::GusApplication --commit'