Bug 898122 - Add some helpful assertions into the JS engine. r=luke

Things go wrong much earlier than the assertion we currently hit.
Bug 898122 - Properly squelch exceptions in FormatFrame. r=luke

Given that this is a debugging tool, we don't really want to just throw in the
towel if we fail to glean all possible information during stack introspection.
Clear the exception and note the failure to the stream.