cmd: Include exit code/reason on failure
cmd: Include exit code/reason on failure

When looking at some logs of udevadm settle failing, there is no
stdout or stderr.  It would be good to know what the exit code is
to see if we can understand this failure better.

Signed-off-by: Tony Asleson <1ef2d381aa56d512126ddaacc29e68136e4fdf55@redhat.com>

stratis-storage/stratisd,trgill/stratisd,stratis-storage/stratisd,stratis-storage/stratisd,trgill/stratisd