Error handling crashers/fixes to roundtrip-test.js

* Error handling code was crashing (!!) :)
* Console-logging error handler was not emitting useful info
  on does-not-exists errors for wiki pages.
* Fixed and tested.

Change-Id: I9a4183761bc79d7c5e577e9485031408d1f6ffe9
Merge "Error handling crashers/fixes to roundtrip-test.js"