extensionSystem: Handle all initExtension() extension errors

The method currently catches errors that occur when calling the
extension's init() method, but throws itself an error if the
expected extension.js file is missing. The former is pointless
if we expect all callers to handle errors themselves anyway, and
we should avoid the latter if we don't - opt for the second option
and handle a missing extension.js file gracefully.

https://bugzilla.gnome.org/show_bug.cgi?id=781728