More verbose errors for the client

[ronkorving]

When edit() throws, it's quite likely that there was an input validation error. It's often vital that an error message is communicated back to the client (eg: "No name was provided"). We don't want to output a stack or anything, but error.message seems very reasonable. Perhaps text/plain is more appropriate here than a JSON object.

If an error object is thrown and it has a code property, we could add a response header: x-error-code for example, so clients can also programmatically deal with these errors.

[axit-joost]

Ooh, I would every much like that. Code for code, messages for people.

[ronkorving]

I kinda expected you would say that. So I went ahead and implemented #22