Issue more precise error when disconnecting a nonexistent connection
Issue more precise error when disconnecting a nonexistent connection

Checks whether the signal exists when issuing an error message when
disconnecting a nonexistent connection. Also prints the callable name.

(cherry picked from commit 6c026a6814eb15c2832f9cfd30ee8ef8d0a236ab)

ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot