Start handling gps solve failures

[sdboyer]

#18 introduces our first gps solve run, which means it's also our first exposure to solve failures. There's a whole menagerie of possibilities there. It's a big enough problem that I've barely even put up issues about it - just sdboyer/gps#20.

I've not invested a ton of time really making errors great in gps so far primarily because it's just hard to do so without a) other people and b) a real tool. glide has provided some of the latter, but less of the former. This is a large problem, though, and making errors comprehensible to users is something we should expect to invest a lot of time in - in this domain, it's one of the main dividing lines between a great and horrible tool.

We've gotta start somewhere, though, so I'm opening this ticket.