lib/oe/package: Improve filedeprunner subprocess handling

lib/oe/package: Improve filedeprunner subprocess handling

Currently the exit code of the spawned program isn't checked so it can
fail and the do_package task will continue merrily upon its way.

Use subprocess.check_output() to ensure we check the exit code and
redirect stderr to stdout so if it fails, we see the error output.

We can then drop the existing exception handling as the subprocess
exception gives a much better error.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>