Use long name for system variables

$! is the same as $OS_ERROR, but I think that OS_ERROR is clearer, and
have been used through the whole OmniPITR source, so changed it to match
style and readability.

It doesn't change how it works, just makes style compatible.

lib/OmniPITR/Program/Archive.pm

@@ -205,7 +205,7 @@ sub make_all_necessary_compressions {
         }
 
         my ( $atime, $mtime ) = ( stat( $self->{ 'segment' } ) )[ 8, 9 ];
-        utime $atime, $mtime, $compressed_filename || $self->log->fatal( 'Error while updating atime/mtime to %s/%s: %s', $atime, $mtime, $! );
+        utime $atime, $mtime, $compressed_filename || $self->log->fatal( 'Error while updating atime/mtime to %s/%s: %s', $atime, $mtime, $OS_ERROR );
 
         $self->{ 'state' }->{ 'compressed' }->{ $compression } = file_md5sum( $compressed_filename );
         $self->save_state();