stbt power: Correct exit status when given invalid outlet

"set -o pipefail" allows unsuccessful exit status of commands to
propagate down a pipeline.

From: http://www.gnu.org/software/bash/manual/bashref.html#The-Set-Builtin

pipefail
    If set, the return value of a pipeline is the value of the
    last (rightmost) command to exit with a non-zero status, or zero if
    all commands in the pipeline exit successfully. This option is
    disabled by default.

Example of the bug:

    $ ./stbt-power --power-outlet ipp:rc.mathembedded.com:p62 status; echo $?
    p62 = OFF
    0
    $ ./stbt-power --power-outlet ipp:rc.mathembedded.com:p62 ono; echo $?
    stbt power: error: invalid command 'ono'
    1
    $ ./stbt-power --power-outlet ipp:rc.mathembedded.com:p654 status; echo $?
    stbt power: error: invalid outlet 'p654' (hint: use the 'status' command)
    0

When "ipp_filteroutlet" called "die", it didn't have the desired effect
because the call to "ipp_filteroutlet" was in a pipeline
("ipp_filteroutlet | head"). There was also a similar problem for the
PDUeX device in function "pdu".