Use a custom exception instead of just a generic exception when plugs are missing

[selkij]

When a plug is missing, a generic system exception is thrown, unless you read the message (which is hidden unless you go to the console tab on VS) you won't get a lot of information, creating a PlugMissingException and PlugNotImplementedException would be better, I can work on it if needed I just need to know the opinion of the dev team

[9xbt]

imo it would be good and would remove a lot of issues in the cosmos github repo