Fix an assertion in web_content_delegates_qt.cpp

If we get a replacement content, we can see a DidFinishLoad event for a frame.
This error page should be ignored based on the frame.

Change-Id: I3e1cd1773e8c5fc608605197c957011ddf258123
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>