Offset Path: an empty result says the offset leaves nothing

With paths selected but every offset empty (an inset larger than the
shape), Offset Path failed with "select paths", sending the user to
look for a selection problem. It now says the offset leaves nothing of
the selected paths; "select paths" stays for selections without paths.

Fixes #1590

crates/engine/src/tests_pathops.rs

@@ -919,3 +919,14 @@ fn trim_merge_and_crop_remove_strokes() {
         }
     }
 }
+
+/// #1590: an inset larger than the shape leaves nothing, and the error says so rather than asking
+/// for paths that are selected.
+#[test]
+fn offset_path_that_leaves_nothing_says_so() {
+    let mut s = session();
+    let a = rect(&mut s, 0.0, 0.0, 10.0, 10.0);
+    s.execute("select.set", &json!({"ids": [a.0]})).unwrap();
+    let e = s.execute("object.path.offsetPath", &json!({"offset": -20})).unwrap_err().to_string();
+    assert!(e.contains("leaves nothing") && !e.contains("select paths"), "{e}");
+}