libfrog: print library errors

libfrog: print library errors

Add a libfrog library function that will print tagged error messages.
This will eliminate the need for a lot of open-coded:

errno = ret;
perror("...");

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>