Adds check for empty configuration to Admin Service stage config endpoint

The Admin Service will now throw an error if the supplied configuration is empty or missing the required `stageClass` parameter. This mostly fixes #245 as the config won't be written to the database, although you will get a syntax error instead - not entirely sure why the attached file breaks the request in that manner.

The Admin Service will now also respond with an error message body of its own when encountering problems, instead of falling back to the container's error page.

For example, a syntax error in the supplied JSON is now a client error, not a server error.