Remove duplicate error() impls in CachedResource subclasses

Remove duplicate error() impls in CachedResource subclasses
https://bugs.webkit.org/show_bug.cgi?id=81161

Reviewed by Alexey Proskuryakov.

No new tests, refactor only.

* loader/cache/CachedCSSStyleSheet.cpp:
* loader/cache/CachedCSSStyleSheet.h:
* loader/cache/CachedFont.cpp:
* loader/cache/CachedFont.h:
* loader/cache/CachedImage.cpp:
* loader/cache/CachedResource.h: Make checkNotify()
     virtual, so the right checkNotify() gets called in error().
* loader/cache/CachedScript.cpp:
* loader/cache/CachedScript.h:
* loader/cache/CachedXSLStyleSheet.cpp:
* loader/cache/CachedXSLStyleSheet.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@112201 268f45cc-cd09-0410-ab3c-d52691b4dbfc