opkg may return non-zero but empty stderr
opkg may return non-zero but empty stderr

This is possible with any version of opkg prior to this fix:
https://bugzilla.yoctoproject.org/show_bug.cgi?id=10407

If this is the case, use stdout in the error message so that an
exception will still be raised.

Signed-off-by: Sergey Kizunov <dfddabaa9af05982b01940b837a87844148069c9@ni.com>

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt