Update error message when trying to switch account with a duplicate email (#3332)

i18n/en.json

@@ -3815,6 +3815,10 @@
     "id": "store.sql_user.update.username_taken.app_error",
     "translation": "This username is already taken. Please choose another."
   },
+  {
+    "id": "store.sql_user.update_auth_data.email_exists.app_error",
+    "translation": "Unable to switch account to {{.Service}}. An account using the email {{.Email}} already exists."
+  },
   {
     "id": "store.sql_user.update_auth_data.app_error",
     "translation": "We couldn't update the auth data"