Fix error message weirdness when processing xincludes that use fallbacks
This patch fixes a relevant bug in bugzilla:Bug 616491 - Error message weirdness when processing xincludes that use fallbacks
ctxt->nbErrors may be bigger than 0 before going in function xmlXIncludeLoadFallback if ctxt has a previous error.
So, "ctxt->nbErrors > 0 ?" cannot be used to determine whether to use fallbcak to process xincludes successfully. This patch tried to fix it.