bitbake-prserv-tool: show error when export file does not exist

* otherwise it shows error about failing import

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
package.bbclass: Ensure package_get_auto_pr is run at the correct point

This needs to run before PKGR and other variables using PRAUTO are used,
including the expansion optimisations.

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