gdbus-tool prints a hint about the expected arguments to a function call
in case of errors.  Unfortunately, it prints this message on all errors.
I've seen this confuse users several times -- they go on tweaking the
arguments trying to get the correct type, even though they had it
correct in the first place.

Let's limit the hint to the case where it was actually invalid arguments
that triggered the problem.  Also, adjust the code that prints the
message so that it will also report on the case that no arguments were
expected.

We could possibly get closer to what we want by comparing the list of
expected arguments with the parameter list, as it was parsed from the
user, but that would involve composing the expected type.  Let's keep
this simple for now.