Fix #2433: Don't fail when a match is only in one arena.

Specifically, in the case where we're requested for the _other_
arena we previously would always hit a KeyError, which would 500
the whole request.

This commit ensures that:
 * we catch such errors if the client explicitly requests an invalid
   match/arena combo so that we 400 suitably.
 * we avoid the error entirely when the client uses the magic next+1
   style requests, and happens to encounter this scenario.

Change-Id: Ia6562b8b375f8af5d57be921d3bb73a03a56ee60
Update the README to be more useful.

Add an introductory paragraph about what this actually is. Move the
usage section up and point developers at the http-dev variant repo
which is more useful. Also note that SRComp has its own requirements
that aren't mentioned here (and may be different).

Change-Id: I4a4376f81db39d0c8a7816ca956a4f0ad5e9710f