#339 clarification of error messages

This PR addresses the 
1. unclarity in ambiguous error method coming from #65 
2. coding hint suggested in #321. Here is chosen to keep the original message, extended with a more simple coding hint then suggested.

 
Btw. did we agree on whether to use FQNs in messages or not? Or is there a separate issue for that? I'd prefer to use the simple name (`String`) or abbreviate the package names (`j.l.String`). The FQN is too much IMO, as the message is shown in the context of an annotated method anyways.