Fix Schrodinger's file existence check in file matcher
Fix Schrodinger's file existence check in file matcher

See: https://stackoverflow.com/a/12518877/1048862

For example, trying to check the existence of "/www/index.php/index.php"
fails but not with an os.IsNotExist()-type error. So we have to assume
that a file that cannot be successfully stat'ed at all does not exist.

abiosoft/caddy,l3dlp/caddy,mholt/caddy,l3dlp/caddy,l3dlp/caddy,abiosoft/caddy,mholt/caddy