LibWeb: Do not crash if parsing failed in load_document()

If `load_document()` is called with a response that has a mime type we
can't use to build a document, we should return nullptr as the spec
says, instead of crashing. Also we should not crash if error happened
during parsing.

Userland/Libraries/LibWeb/HTML/Navigable.cpp

@@ -813,8 +813,6 @@ WebIDL::ExceptionOr<void> Navigable::populate_session_history_entry_document(JS:
 
             // 2. If document is null, then run completionSteps and return.
             if (!document) {
-                VERIFY_NOT_REACHED();
-
                 completion_steps();
                 return;
             }