Add a test to ensure untyped exceptions are caught.
Allow for the case where nsIScriptError objects have no sourceName.
The untyped exceptions don't get sourceName values... which is sad. Very sad.

Even sadder is that Mozilla seems to have ignored adding any ability to get a stack trace out of an nsIScriptError since as early as 2002...

* https://bugzilla.mozilla.org/show_bug.cgi?id=125612
* https://bugzilla.mozilla.org/show_bug.cgi?id=814497
* https://bugzilla.mozilla.org/show_bug.cgi?id=493414

Bah.
Firefox doesn't log the 'Error:' part of a thrown error. Also, sourceName ends up being an empty string, not 'null'.
Merge pull request #2 from chrisdarroch/fixup

Log untyped errors