Improving LDAP user creation fail error message (#4481)

i18n/en.json

@@ -2675,6 +2675,10 @@
     "id": "ent.ldap.disabled.app_error",
     "translation": "AD/LDAP disabled or licence does not support AD/LDAP."
   },
+  {
+    "id": "ent.ldap.create_fail",
+    "translation": "Unable to create LDAP user."
+  },
   {
     "id": "ent.ldap.do_login.bind_admin_user.app_error",
     "translation": "Unable to bind to AD/LDAP server. Check BindUsername and BindPassword."