Fix display of server-provided validation error when adding subscriber

no issue
- make the check for "email" in the server provided error case-insensitive

core/client/app/components/modals/new-subscriber.js

@@ -18,7 +18,7 @@ export default ModalComponent.extend({
                 this.send('closeModal');
             }).catch((errors) => {
                 let [error] = errors;
-                if (error && error.match(/email/)) {
+                if (error && error.match(/email/i)) {
                     this.get('model.errors').add('email', error);
                     this.get('model.hasValidated').pushObject('email');
                 }

core/client/app/mirage/config.js

@@ -60,8 +60,8 @@ function mockSubscribers(server) {
         if (subscriber) {
             return new Mirage.Response(422, {}, {
                 errors: [{
-                    errorType: 'DataImportError',
-                    message: 'duplicate email',
+                    errorType: 'ValidationError',
+                    message: 'Email already exists.',
                     property: 'email'
                 }]
             });

core/client/tests/acceptance/subscribers-test.js

@@ -177,7 +177,7 @@ describe('Acceptance: Subscribers', function() {
             andThen(function () {
                 // the validation error is displayed
                 expect(find('.fullscreen-modal .error .response').text().trim(), 'duplicate email validation')
-                    .to.match(/duplicate/);
+                    .to.equal('Email already exists.');
 
                 // the subscriber is not added to the table
                 expect(find('.lt-cell:contains(test@example.com)').length, 'number of "test@example.com rows"')