acme.c: Exit with an error if path record validation fails

ib_acme utility returns non zero value if path validation
fails. Use of exit code allows ib_acme to be used by an
automatic script.

Signed-off-by: Sasha Kotchubievsky <sashakot@mellanox.com>
Signed-off-by: Hal Rosenstock <hal@mellanox.com>