Use an error message closer to old dumpfs(8) in the case of truncated/no

superblock.

Submitted by:	kkenn

Can't use it verbatim, at least I hate to, as the ", skipped" bit doesn't
make much sense in a library, to me.

lib/libufs/sblock.c

@@ -60,7 +60,7 @@ sbread(struct uufsd *disk)
 
 	for (sb = 0; (superblock = superblocks[sb]) != -1; sb++) {
 		if (bread(disk, superblock, disk->d_sb, SBLOCKSIZE) == -1) {
-			disk->d_error = "truncated superblock";
+			disk->d_error = "non-existent or truncated superblock";
 			DEBUG(NULL);
 			return -1;
 		}