partx: exit with error code when partition read failed

partx: exit with error code when partition read failed

Make sure partx exits with a non-0 return code when
it runs into either code-path where getting the partition
table failed (or wasn't even attempted because of previous
error condition).

Change was tested using:
touch /tmp/foobar
partx -s - /tmp/foobar

Previously that was only printing an error/warning message
and then exiting with 0, but after this change it exits
with 1.

Signed-off-by: Andreas Henriksson <andreas@fatal.se>
Reported-by: Juan Céspedes <cespedes@debian.org>
Addresses: https://bugs.debian.org/898426