500 response on bogus operation parameter

[dbyron0]

One of the calls to getOperation in ScepServlet.java is wrapped in a try/catch block looking for IllegalArgumentException. Unfortunately there's an earlier call in getMessageBytes that isn't. As a result, when hitting the servlet with a bogus operation parameter, the response has status code 500 from the unhandled exception, instead of 400 with the Invalid "operation" parameter response body.

[dbyron0]

See #88 for a crack at fixing this.