Store 'str' as the error message to be returned instead of the default
message setup via add_action(). This is the preferred way to display
error messages since it allows other objects a chance to respond to the
same verb (command). Do not use write() to display the error message
since this will require you to return a 1 (unless you want to see the
result of the write() in addition to the 'What?' message). However, if
you do return a 1, then no other objects will get a chance to respond
to the user command.
If a function is passed instead of a string, the function is called
instead of printing a message. If the function returns a string, that
string is used as the failure message. Also, this_player() is set cor‐
rectly, so write() can be used.
If "notify_fail()" is called more than once, only the last call will
have an effect.
The idea behind this function is to allow better error messages than