Add call validation to CommandPalette

[KoBeWi]

Code from confused user™:

@tool
extends EditorScript

func bundle_audio():
	print("Hello World")
	
func _run():
	var command_palette = get_editor_interface().get_command_palette()

	var name = "audio/bundle"
	command_palette.remove_command(name)
	
	command_palette.add_command("Bundle Audio", name, bundle_audio) 

	print("command added")

The EditorScript instance is gone before the command is run, so it can't be called. However editor palette doesn't do any call validation and it just fails silently.

This PR adds error messages:

[YuriSizov]

I would probably use a toast for this. I don't see a reason to raise a blocking popup. But otherwise seems like a good idea!

[KoBeWi]

[akien-mga]

Fails building:

In file included from ./core/object/method_bind.h:34,
                 from ./core/object/class_db.h:34,
                 from ./core/object/ref_counted.h:34,
                 from ./core/io/resource_uid.h:34,
                 from ./core/io/resource.h:34,
                 from ./core/input/input_event.h:35,
                 from ./core/input/shortcut.h:34,
                 from ./editor/editor_command_palette.h:34,
                 from editor/editor_command_palette.cpp:31:
./core/variant/binder_common.h: In instantiation of 'void call_with_variant_args_helper(T*, void (T::*)(P ...), const Variant**, Callable::CallError&, IndexSequence<Is ...>) [with T = EditorCommandPalette; P = {String&}; long unsigned int ...Is = {0}]':
./core/variant/binder_common.h:417:40:   required from 'void call_with_variant_args(T*, void (T::*)(P ...), const Variant**, int, Callable::CallError&) [with T = EditorCommandPalette; P = {String&}]'
./core/object/callable_method_pointer.h:104:25:   required from 'void CallableCustomMethodPointer<T, P>::call(const Variant**, int, Variant&, Callable::CallError&) const [with T = EditorCommandPalette; P = {String&}]'
./core/object/callable_method_pointer.h:100:15:   required from here
Error: ./core/variant/binder_common.h:303:25: error: cannot bind non-const lvalue reference of type 'String&' to an rvalue of type 'String'
  303 |  (p_instance->*p_method)(VariantCasterAndValidate<P>::cast(p_args, Is, r_error)...);
      |  ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[KoBeWi]

I know, but I can't decipher the error.

[akien-mga]

execute_command takes a String &, but you're passing a String.
Apparently the direct call syntax handles that fine but callable_mp is more picky.

[KoBeWi]

I added const and it magically works now.

[akien-mga]

Thanks!