Do not return false from onerror handler. Fixes #27

Since bugsense's handler do not need to suppress error handling there is no need to return anything from onerror handler.
Older Opera and Chrome have onerror handler return value semantics inverted:
http://stackoverflow.com/questions/8087240/if-i-override-window-onerror-in-javascript-should-i-return-true-or-false
Therefore, it's even safer to avoid returning anything from onerror.