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
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-Jerome.Python2

00:22:23

passed SlapOS.Eggs.UnitTest-Jerome.Python3

00:13:46

passed SlapOS.Eggs.UnitTest-Master.Python2

00:12:48

passed SlapOS.Eggs.UnitTest-Master.Python3

00:13:56

passed SlapOS.Eggs.UnitTest-Jerome.Python2

00:16:04

passed SlapOS.Eggs.UnitTest-Jerome.Python3

00:17:04

passed SlapOS.Eggs.UnitTest-Master.Python2

00:23:30

passed SlapOS.Eggs.UnitTest-Master.Python2

00:12:37

passed SlapOS.Eggs.UnitTest-Master.Python2

00:24:02

passed SlapOS.Eggs.UnitTest-Master.Python3

00:13:33

passed SlapOS.Eggs.UnitTest-Master.Python3

00:13:02

passed SlapOS.Eggs.UnitTest-Master.Python3

00:14:43