pxar: encoder: warn on lacking read permissions instead of fail.

pxar: encoder: warn on lacking read permissions instead of fail.

If during creation of the archive, files/dirs with lacking read permissions are
encountered, the user is displayed a warning and the archive is created without
including the file/dir.
Previously this resulted in an error and the archive creation failed.

In order to implement this also for the .pxarexclude files, the Error type of
MatchPattern::from_file() and MatchPattern::from_line() was adopted accordingly.

Signed-off-by: Christian Ebner <c.ebner@proxmox.com>