bectl: Improve error message when ZFS root is not found.

When recovering a system that is unbootable due to some
problem with the active BE, it is likely you'll be booted
from a rescue image running UFS.  In this case, bectl
needs help finding the zpool root that you want to operate
on.  In this case, improve the error message to suggest
specifying a root, rather than just emitting a generic
error message that might imply, to the naive user, that
there is a ZFS compatibility issue between the rescue
image and the on-disk ZFS pool.

Reviewed by: imp, kevans
Sponsored by: Netflix
Differential Revision:	https://reviews.freebsd.org/D39346

sbin/bectl/bectl.c

@@ -584,9 +584,13 @@ main(int argc, char *argv[])
 	}
 
 	if ((be = libbe_init(root)) == NULL) {
-		if (!cmd->silent)
+		if (!cmd->silent) {
 			fprintf(stderr, "libbe_init(\"%s\") failed.\n",
 			    root != NULL ? root : "");
+			if (root == NULL)
+				fprintf(stderr,
+				    "Try specifying ZFS root using -r.\n");
+		}
 		return (-1);
 	}