jsduck: Add 'mediawiki.startUp.js' to eg-iframe
Follows-up 2ea420eed86. Without this, the startUp() is never called,
and mw.config remains undefined which caused some of the examples
to no longer work properly.
Also move onerror handler to first <script> (follows-up a64a07d).
Exceptions from other code (eg. startup or mediawiki modules),
were not being caught.
Also include 'error' in the line being logged from onerror.