Upstreaming: Customize the suppress-warnings prefix that is included in error messages

[cpovirk]

28089b9

Obviously this is minor, and obviously it is sometimes nice to be able to suppress only a more fine-grained category. For the moment, though, I am erred on the side of making the output shorter and making it clear how to suppress. If we end up wanting to stick with that approach, we'd want to upstream.

There may be other ways around this: Maybe we'd plug in our own Messager that munges the default message.

(Note that it is not sufficient to define a single message key, nullness=%s, and then use it for all errors we report. That's because CF itself issues errors with its own keys.)