Fix POST error message.
Fix POST error message.

Change-Id: Ib0b4df01cbf21ec9b8d74badc43c3315adf12cdc

agoode/cups-connector,201371192/cups-connector,agoode/cloud-print-connector,agoode/cloud-print-connector,avlis/cloud-print-connector,swills/cups-connector,google/cloud-print-connector,agoode/cloud-print-connector,google/cloud-print-connector,agoode/cups-connector,agoode/cloud-print-connector,google/cloud-print-connector,avlis/cloud-print-connector,google/cloud-print-connector,avlis/cloud-print-connector,201371192/cups-connector,avlis/cloud-print-connector,swills/cups-connector