Fixed #19987 -- Disabled host validation when DEBUG=True.

The documentation promises that host validation is disabled when
DEBUG=True, that all hostnames are accepted. Domains not compliant with
RFC 1034/1035 were however being validated, this validation has now been
removed when DEBUG=True.

Additionally, when DEBUG=False a more detailed SuspiciousOperation
exception message is provided when host validation fails because the
hostname is not RFC 1034/1035 compliant.