Werk #17334: Catch invalid "Explicit hosts" condition on save instead of crashing

When invalid host names were passed into the "Explicit hosts" condition while creating or editing a rule, Checkmk would crash with the message `Internal error: Invalid hostaddress: '{explicit hosts condition}'.

With this werk, the form will warn you that you have entered an invalid Explicit hosts condition. This allows you to fix the error without losing the rest of the entered inputs.

To the list of all Werks