Improve errors for add_to_group and remove_from_group, improve documentation

[nathanfranke]

Tested and seems to work as expected, including release builds

$ ./test.x86_64 
Godot Engine v4.0.alpha.custom_build.c80540f9a - https://godotengine.org
Vulkan API 1.2.0 - Using Vulkan Device #0: AMD - AMD Radeon RX 6800 XT
 
ERROR: Node is already in group 'a'.
   at: add_to_group (scene/main/node.cpp:1749)
ERROR: Node is not in group 'a'.
   at: remove_from_group (scene/main/node.cpp:1766)

At first I assumed the ERR_* macros were removed in release builds, but I guess they aren't.

[Calinou]

At first I assumed the ERR_* macros were removed in release builds, but I guess they aren't.

Since 3.4, error messages are included in release builds to make error reporting from users more relevant: #53405

[nathanfranke]

Since 3.4, error messages are included in release builds to make error reporting from users more relevant: #53405

Looks like that PR only effects error messages, I am more concerned with whether the function will return if COND is true.

[MewPurPur]

Is this still relevant?

[Mickeon]

I believe this PR is no longer relevant, since neither of these methods generate errors anymore since #68564 and their docs have been changed in #68560. Should this be closed?