PLT-2976 Adding checkout for missing SMTP server when testing email connection (#3115)

* PLT-2976 Adding checkout for missing SMTP server when testing email connection

* Fixing unit test

i18n/en.json

@@ -83,6 +83,10 @@
     "id": "api.admin.test_email.subject",
     "translation": "Mattermost - Testing Email Settings"
   },
+  {
+    "id": "api.admin.test_email.missing_server",
+    "translation": "SMTP Server is required"
+  },
   {
     "id": "api.admin.upload_brand_image.array.app_error",
     "translation": "Empty array under 'image' in request"