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.
Similar to `weldr/json.go`, this file will contain structs that are
shared between client and server and only meant to be used to
serialize from and to JSON.
This immediate change allows us to make fields in `Job` private in a
future commit.
