contrib: more clear error msg when .sum-xxx doesn't exist

Make clear message when a .sum-xxx target is not
defined in the rule.mak file of a contrib.
The old message wasn't explicit about the target name and was
not clear that "check" was the .sum-xxx target.

Change-Id: Ic7fba0881170f825ee79f13613f190a0e5851308
Reviewed-by: Stepan Salenikovich <stepan.salenikovich@savoirfairelinux.com>