Improve differing sector size error

When adding or replacing a vdev with a different sector size the
error message should be more useful.  In addition to describing
the problem provide a hint that the '-o ashift' option can be
used to override the optimal default value.

Since using a non-optimal value may incur a significant performance
penalty we should issue this error.  But there a numerous reasons
why a administrator may wish to do this anyway.

Signed-off-by: Niklas Edmundsson <ZNikke@github>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2421

lib/libzfs/libzfs_pool.c

@@ -2753,10 +2753,11 @@ zpool_vdev_attach(zpool_handle_t *zhp,
 
 	case EDOM:
 		/*
-		 * The new device has a different alignment requirement.
+		 * The new device has a different optimal sector size.
 		 */
 		zfs_error_aux(hdl, dgettext(TEXT_DOMAIN,
-		    "devices have different sector alignment"));
+		    "new device has a different optimal sector size; use the "
+		    "option '-o ashift=N' to override the optimal size"));
 		(void) zfs_error(hdl, EZFS_BADDEV, msg);
 		break;