Return "Bad Request" for errors caused by client

[schuetzm]

Currently, enforce() is used to check for invalid requests to the server, for example malformed HTTP queries, or missing/wrongly typed query parameters in APIs. enforce() throws a generic Exception, which makes the server return 500 Internal Server Error to the client. However, the appropriate response for invalid requests is 400 Bad Request.

[s-ludwig]

Thanks, makes sense. The only thing that is a bit ugly is the two overloads of enforceHTTP that only differ in the order of their arguments. Maybe instead of making the status parameter a default one, it would be better to introduce a separate enforceHTTPRequest(cond, message) (any better name?) function?

[schuetzm]

I renamed it to enforceBadRequest.

[s-ludwig]

Thanks! Looks like the commit is based on an old failing commit. Everything looks fine though, so I'll just merge like it is.