ConnectionDialog: Don't allow connecting to a Node without a script if target method is invalid.
ConnectionDialog: Don't allow connecting to a Node without a script if target method is invalid.

Shows a warning now.
Fixes #6656

(cherry picked from commit 5b7021434f27145550ddb5de74dc66e9273d4dcb)

pixelpicosean/my-godot-2.1,pixelpicosean/my-godot-2.1,pixelpicosean/my-godot-2.1,pixelpicosean/my-godot-2.1,pixelpicosean/my-godot-2.1