vpc_open() merely prints a warning when it finds a bad header
checksum.  Turn that into a hard error.

Cc: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20181017082702.5581-39-armbru@redhat.com>
[Error message capitalized for local consistency]
Reviewed-by: Kevin Wolf <kwolf@redhat.com>