Provide more details when keys not found.

A request was made to provide the original key as well as the member
that was not found. In compound keys {{Customer.Address.ZipCode}}, any
of the names could be invalid, but knowing the fully-qualified name is
useful for error handling.

I also detected a bug where the code wasn't handling the case that a
sub-key didn't exist. This needs to be handled using the KeyNotFound
event.