The API is always advertising a content type of application/json, but
not sending JSON on errors.
Change it to send simple JSON objects like this:
{ "message": "something went wrong" }
This can be extended to include more structured information in the
future.
Also return an (for now) empty JSON object from `addJobHandler()`. It
returned nothing before, which is invalid JSON.
Stop testing for the actual error strings in `api_test.go`. These are
meant for humans and can change. Only check what a client could
meaningfully check for, which is only the HTTP status code right now.
8ef4db816d