backend/ipp: Change handling of job_canceled

backend/ipp: Change handling of job_canceled

job_canceled can be -1, 0, or 1. 0 indicates no error, 1 indicates
that we were cancelled in normal operations (user cancelled from
computer) and -1 indicates cancelled by printer.  There is clearly
an assumption that -1 is false since there is a check for
job_canceled < 0 a few lines later.

BUG=b:174793115
TEST=Print to printer with malformed PWG-Raster content

Change-Id: I88f504d00e0680507cd439b633c6c288cf7c5390