Show unmatched stubs in `VerifyStubs` error output

[twelvelabs]

The VerifyStubs methods in the api and run packages just list the number of unmatched stubs. It would be a better UX if they actually listed the match strings. Something like:

found 2 unmatched stub(s):
- MatchPost: /api/v1/users 
- MatchGet: /api/v1/users/123

// or...

found 2 unmatched stub(s):
- MatchString: /bin/echo foo
- MatchString: /bin/echo bar

[twelvelabs]

Unfortunately stubs don't have access to any of this data due to the way matchers are created. Typically we use a factory function (for example, MatchPost) to create an anonymous Matcher function that gets passed to the stub, so even if we tried to use reflection shenanigans we wouldn't get anything useful to show users.

Going to keep this open for a bit in case we ever redo how matchers work.