<rdar://problem/5048818> - REGRESSION: Incompletely loaded resources being saved to the object cache

        Due to a subtle change in loader behavior back in 10904, we would stop all loaders before calling 
        didFail() on them in the Cache loader.  As a result, we basically cleared all of the Subresource Loaders 
        out of the Cache loader before more properly failing them as errored out.  The result?  Partially loaded
        resources being cached.
        
        Since Loader::didFail() both calls error() on the object *and* removes the loader, the solution is to call 
        didFail() for all cancelled loaders instead of *only* removing them from the set of active loaders.
        
        In addition, pages that didn't completely load were being saved to the back/forward cache.  To fix that,
        I added a null check on the DocumentLoader's error to see if the page ended in an error, or did indeed 
        completely load.

        Note that the layout test for this - if possible - will require other enhancements including possibly adding
        support for window.stop().  That task is documented in <rdar://problem/5061826>

        * loader/FrameLoader.cpp:
        (WebCore::FrameLoader::provisionalLoadStarted): Fixed a few bugs relating to my original BFCache rewrite to
          more perfectly restore the original behavior - including only caching HTML documents via the 
          m_client->canCachePage() call
        (WebCore::FrameLoader::canCachePage): Don't make the call to m_client->canCachePage() as that serves a different
          purpose
          - Check the mainDocumentError to see if the load ended in error as a further criteria in determining the 
          cachability of a page

        * loader/loader.cpp:
        (WebCore::Loader::cancelRequests): Call didFail(cancelledError()) instead of just removing the loaders from the 
          loaders-in-progress set.  This adds the effect of properly cleaning up the cached object.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@20178 268f45cc-cd09-0410-ab3c-d52691b4dbfc