caddy-frontend: Explain custom_domain a bit better

Users are often providing wrong values which are rejected later. So give the
clear information to the user.
32 jobs for master in 0 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-Master.Python2

passed SlapOS.Eggs.UnitTest-Master.Python3

00:14:31

passed SlapOS.SlapRunner.ResilienceTest-Master

00:23:39

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:26:39

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

07:48:26

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

04:00:30

failed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

15:03:26

passed SlapOS.Eggs.UnitTest-Master.Python2

passed SlapOS.Eggs.UnitTest-Master.Python2

passed SlapOS.Eggs.UnitTest-Master.Python2

00:35:02

passed SlapOS.Eggs.UnitTest-Master.Python2

00:12:05

passed SlapOS.Eggs.UnitTest-Master.Python2

passed SlapOS.Eggs.UnitTest-Master.Python2

00:00:00

passed SlapOS.Eggs.UnitTest-Master.Python2

passed SlapOS.Eggs.UnitTest-Master.Python2

00:00:25

passed SlapOS.Eggs.UnitTest-Master.Python2

00:09:05

passed SlapOS.Eggs.UnitTest-Master.Python3

passed SlapOS.Eggs.UnitTest-Master.Python3

00:11:17

passed SlapOS.Eggs.UnitTest-Master.Python3

passed SlapOS.Eggs.UnitTest-Master.Python3

00:32:36

passed SlapOS.Eggs.UnitTest-Master.Python3

passed SlapOS.Eggs.UnitTest-Master.Python3

passed SlapOS.Eggs.UnitTest-Master.Python3

passed SlapOS.Eggs.UnitTest-Master.Python3

passed SlapOS.Eggs.UnitTest-Master.Python3

passed SlapOS.SlapRunner.ResilienceTest-Master

00:22:28

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:20:11

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:23:16

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

06:55:25

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

05:44:28

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

03:26:27

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

05:37:47