Add better error description for case when we are doing resize and

scheme-specific method returns EBUSY.

MFC after:	1 week

sys/geom/part/g_part.c

@@ -1316,7 +1316,9 @@ g_part_ctl_resize(struct gctl_req *req, struct g_part_parms *gpp)
 
 	error = G_PART_RESIZE(table, entry, gpp);
 	if (error) {
-		gctl_error(req, "%d", error);
+		gctl_error(req, "%d%s", error, error != EBUSY ? "":
+		    " resizing will lead to unexpected shrinking"
+		    " due to alignment");
 		return (error);
 	}