Fixed a lack of an error message in draw.js.

craftscripts/draw.js

@@ -78,7 +78,10 @@ var sess = context.remember();
 
 context.checkArgs(1, 1, "<image>");
 
-var img = ImageIO.read(context.getSafeFile("drawings", argv[1]));
+var f = context.getSafeFile("drawings", argv[1]);
+
+if (f != null) {
+    var img = ImageIO.read(f);
 
     var width = img.getWidth();
     var height = img.getHeight();
@@ -92,3 +95,4 @@ for (var x = 0; x < width; x++) {
             sess.setBlock(origin.add(x, 0, y), new BaseBlock(35, data));
         }
     }
+}