Demote unavailability of dmraid from error to warning only

Demote unavailability of dmraid from error to warning only

dmraid had its latest Debian upload in 2017, the last upstream release
dates back to 2010 (see http://people.redhat.com/~heinzm/sw/dmraid/src/)
and it has plenty of unresolved bugs. It's furthermore relevant for so
called fake RAIDs only, something which is rather exotic and therefore
Grml ISOs not including dmraid should just raise a warning, rather than
an error message.

Thanks to Sipwise for sponsoring my development time.
See Sipwise internal ticket TT#121951