[BUG] Throwing unhandled rejection errors when Firefox navigates to neterr page

[JoelEinbinder]

Sometimes Firefox wants to navigate to an internal aboutNetError.xhtml. We have an assert than fails in this case, because the new documentId is empty string and doesn't match the pending document id.

[aslushnikov]

@JoelEinbinder any chance you can add a test for this?

[pavelfeldman]

No reports from the users so far.