Update errors to USVString (#755)

Fixes #754

Author: npm1

spec/index.bs

@@ -627,7 +627,7 @@ to use a federated account.
 <pre class="idl">
 dictionary IdentityCredentialErrorInit {
   DOMString error;
-  DOMString url;
+  USVString url;
 };
 </pre>
 
@@ -636,7 +636,7 @@ dictionary IdentityCredentialErrorInit {
   interface IdentityCredentialError : DOMException {
     constructor(optional DOMString message = "", optional IdentityCredentialErrorInit options = {});
     readonly attribute DOMString error;
-    readonly attribute DOMString url;
+    readonly attribute USVString url;
   };
 </pre>