libalpm: check filecache_find return and log errors

Some user had erros while updating their system.

:: Proceed with installation? [Y/n] :: Retrieving packages... checking keyring... checking package integrity... error: failed to commit transaction (invalid or corrupted package) Errors occurred, no packages were upgraded.

The issue was filecache_find returning null and alpm passing that null path to check validity. How this happened I have no idea. It may be something to do with the user's cachedir being a network drive.

Also warn when the file exists but it is not a regular file or can not be opened.