irc: fix dropped errors

[alrs]

This fixes two dropped err variables in the irc package.

[alrs]

Does this need anything else, @slingamn?

[slingamn]

I think there is a potential change of behavior here since if the key is not found, we want errCasFailed but this will cause it to return buntdb.ErrNotFound instead.

[slingamn]

The most defensive implementation would probably be:

		if err != nil || credStr != curCredStr {
			return errCASFailed
		}

[slingamn]

This will get refactored in #1986