Fixes <rdar://problem/4237479> REGRESSION (Cambridge-Denver): old QuickTime movie continues to play sound after reload
        We were adding the movie to the document twice after the changes were added to handle fallback content.
        There are some errors for which we should not render fall back content

        * Misc.subproj/WebKitErrorsPrivate.h:
        Introduced WebKitErrorPlugInWillHandleLoad to represent the cancel we do
        to prevent loading plugin content twice

        * Plugins.subproj/WebPluginDocumentView.m:
        (-[WebPluginDocumentView dataSourceUpdated:]): Ditto

        * WebView.subproj/WebDataSource.m:
        (-[WebDataSource _receivedMainResourceError:complete:]):
        Don't handleFallbackContent on WebKitErrorPlugInWillHandleLoad or on a user cancel


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@10537 268f45cc-cd09-0410-ab3c-d52691b4dbfc