Print spooler should not crash if fed non-PDF content.

It is possible that a buggy app breaks the contract and provides
content to be printed in format other than PDF. This was leading
to a crash in the print spooler. This change fixes the crash and
shows a user friendly error message.

bug:17642690

Change-Id: I5a4acb06080a152562655da6851467b3e71d8658