Removes the error message when the network peer is not valid and returns false instead.
This makes it simpler to make games that are both on/offline by replacing server checks of
'''
if is_instance_valid(get_tree().network_peer) and get_tree().is_network_server():
# Do server things
'''
with
'''
if get_tree().is_network_server():
# Do server things
'''
Requires no changes to the docs because both the MultiplayerAPI and SceneTree docs don't mention the error.
74379b15ff