Make Reverse geocoder stop using bbox

Fixes #183 on github
@jwalgran and I discussed this and I *think* this was the agreed outcome. Logging a stacktrace and custom message is not so silent.

Since this falls through to an http geocoder that doesn't need these values, it's nice not to totally break geocoding when these values are missing.

That said, there can be some friction/duplication potential in the way this is selectively validating instance config data in the context of when it is used, not when it is received. Thoughts on that?