When one of the parameters to DispatcherBase::Invoke is incorrect, you
get an error in a strange place. It is deep in a call stack using some
heavily templated types rather than where the Invoke is actually called.

Formerly, the code was structured to give a very obfuscated error
message. Try to make this easier on users by first adding helpful hints
in the code around where the error is to explain why the error occured
and helpful advice on how to track down the problem. Second, reconfigure
the static_assert so that the compiler gives a readable error message.
Third, add some auxiliary error messages that provide additional
information about the types and classes involved in the error.