Bug 1812696 - Implement FormData constructor submitter parameter r=webidl,smaug

Also improve error messages for current submitter validations

Spec PR: https://github.com/whatwg/xhr/pull/366
WPT PR: https://github.com/web-platform-tests/wpt/pull/37895

Differential Revision: https://phabricator.services.mozilla.com/D167576

dom/html/HTMLFormElement.cpp

@@ -296,7 +296,7 @@ void HTMLFormElement::RequestSubmit(nsGenericHTMLElement* aSubmitter,
     // 1.2. If submitter's form owner is not this form element, then throw a
     //      "NotFoundError" DOMException.
     if (fc->GetForm() != this) {
-      aRv.Throw(NS_ERROR_DOM_NOT_FOUND_ERR);
+      aRv.ThrowNotFoundError("The submitter is not owned by this form.");
       return;
     }
   }