Show warnings for invalid tokens in synopsis

If you add a `@synopsis` tag to your command, you would expect it to complain if you got the syntax wrong, since it affects the command's behavior.

The synopsis stopped being just a documentation aid ever since we started doing argument validation based on it. 

[Later edit: Actually, the `@synopsis` annotation was used for doing arg validation right from the start: #178]

Brought up by #633.