Sometimes 'iscsiadm -m session' outputs warnings on stderr, but
still serves its purpose (usable stdout). We should not give up on
it when stderr is non-empty - instead, rely on the exit status, log
the stderr as a warning, and move on.
This change also removes 1 (ISCSI_ERR) from the list of acceptable
exit codes for the iscsiadm command, to ensure that failures get
caught.
Change-Id: Id8183cf3d8baf2f8ba6a00a47fa2ad7cc2a96aa5
Closes-Bug: #1732199
d398fa8233