core: Alert if priorities were overridden

During buildSequence, warn or optionally abort, if the final sequence
has methods ordered in reverse to their priorities. This might happen if
the priorities conflict with before/after.

Change-Id: I5610d172ee6dd8765beefb8b27a9a94395b232e2
Signed-off-by: Yedidyah Bar David <didi@redhat.com>