api: client crash logging

[seabo]

Create an endpoint for logging Python client crashes.

I think the simplest and cleanest solution to this is to just have a PUT /clientCrashLog endpoint. This is basically just a big text body with (perhaps) a few lines of contextual data followed by the stacktrace. It should still have the API key auth header, so we only save data that has originated from someone with a HitSave account (to make it at least a bit harder for someone to fill up our S3 with spurious crap). The API key (or some user identifier) should also be included in the body content, so we can easily see which user the crash happened to.