more engraving with empty wands (#Q85)

#Q85: Bug: wrest a wand in one turn through engraving
From a bug report:  engraving with an empty wand while not levitating, then using
ESC to abort when asked for the text to write, let you attempt to wrest
the last charge without using up a turn.  Between the time this report was
sent and received, someone else reported similar issue with levitation,
where no prompting occurs.  The fix for it also fixed the original problem.
Somewhat inadvertently though, since you can now no longer write in the
dust with a empty wand.  I'm going to leave it this way; all that's needed
is some feedback about failing to engrave.

src/engrave.c

@@ -774,8 +774,14 @@ doengrave()
 		} else { /* end if zappable */
 		    /* failing to wrest one last charge takes time */
 		    ptext = FALSE;	/* use "early exit" below, return 1 */
-		    /* cancelled wand turns to dust unless hero can't write */
-		    if (otmp->spe < 0 && can_reach_floor(TRUE)) zapwand = TRUE;
+		    /* give feedback here if we won't be getting the
+		      "can't reach floor" message below */
+		    if (can_reach_floor(TRUE)) {
+			/* cancelled wand turns to dust */
+			if (otmp->spe < 0) zapwand = TRUE;
+			/* empty wand just doesn't write */
+			else pline_The("wand is too worn out to engrave.");
+		    }
 		}
 		break;