Issue 9265 - assertNotThrown should emit msg from thrown exception if available.

http://d.puremagic.com/issues/show_bug.cgi?id=9625

This will print either the user-specified message, the thrown message, or a period if neither are available (in that order). 

This makes it simpler to investigate why an `assertNotThrown` has failed when used with exceptions that have their `msg` field set.