Return 400 on invalid request, rather than 500.
Return 400 on invalid request, rather than 500.

Currently a request body which is valid JSON but doesn't convert to a
Python dict will cause a TypeError and return 500. Return a 400 error
instead.

auvsi-suas/interop,auvsi-suas/interop,auvsi-suas/interop,auvsi-suas/interop