ChangeEmailModal: dismiss alert on new request

Removing old errors at the beginning of the next request, rather than at the end of the next successful request, makes it clearer that any new errors are caused by the new inputs.

See #2467 (comment)

js/src/forum/components/ChangeEmailModal.js

@@ -109,6 +109,7 @@ export default class ChangeEmailModal extends Modal {
     const oldEmail = app.session.user.email();
 
     this.loading = true;
+    this.alertAttrs = null;
 
     app.session.user
       .save(
@@ -120,7 +121,6 @@ export default class ChangeEmailModal extends Modal {
       )
       .then(() => {
         this.success = true;
-        this.alertAttrs = null;
       })
       .catch(() => {})
       .then(this.loaded.bind(this));