Calling file-notify-rm-watch on an invalid descriptor may throw a
invalid-function error, but it should throw a file-notify-error instead.

For example, this may occur when calling the function multiple times on
the same descriptor.

The error stems from file-notify--rm-descriptor, which contains the expression

      (funcall
       (cdr (assoc file (cdr registered)))
       `(,descriptor stopped ,(if file (expand-file-name file dir)
       dir)))

, but (assoc file (cdr registered)) is nil.  I think this error is
confusing.

Attached you'll find a test case for this.

-ap