Let patchdir path be absolute

Signed-off-by: Jani Monoses <jani@ubuntu.com>
Exit with error message on inexistent patch directory

Signed-off-by: Jani Monoses <jani@ubuntu.com>
Only pass one arg at a time to dirname, to fix script on 12.04

Signed-off-by: Jani Monoses <jani@ubuntu.com>