Improve assertion reports using "produced" message sets.

[jmalloc]

This assertion logic was written before handlers were required to declare the message types they produce. Now that such information is available, the explanation messages such as:

• no relevant handlers (aggregate or integration) were engaged
• no relevant handlers (process) were engaged

Could be improved by referring to the specific handler that would produce the expected message.

Additionally the suggestions like:

• verify the logic within the '<process>' process message handler

Could show a more specific message if the handler is the one that would produce the expected message.