test_check_url_available: adjust message for requests 2.27.0

Improved error text for misnamed InvalidSchema and MissingSchema exceptions.

https://github.com/psf/requests/blob/v2.27.0/HISTORY.md#2270-2022-01-03

slapos/test/promise/plugin/test_check_url_available.py

     self.assertEqual(result['result']['failed'], True)
     self.assertEqual(
       result['result']['message'],
-      "ERROR: Invalid URL '': No schema supplied. Perhaps you meant http://?"
+      "ERROR: Invalid URL '': No scheme supplied. Perhaps you meant http://?"
     )
 
   def test_check_url_site_off(self):

[Jérome Perrin]

needed for slapos!1168 (closed)