gdk-pixbuf: handle postinstall errors differently

gdk-pixbuf: handle postinstall errors differently

Apparently gdk-pixbuf-query-loaders always returns 0 but writes any
errors to stderr. So, the conditional exit at the end of the command was
useless. Write the errors to a file and exit only if the file size is
greater than 0.

(From OE-Core rev: 901fbfac48918db7bf809ded3df533504141c5ca)

Signed-off-by: Laurentiu Palcu <laurentiu.palcu@intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>