Improve error messaging when types are not exported

[wesbillman]

When a user tries to ftl.call a verb that is not exported, it isn't obvious to the user what they need to do to resolve the error. Should we add a message like

Currently the message(s) are:

call first argument must be a function but is an unresolved reference to time.Internal
call first argument must be a function in an ftl module

Should we update to something like:

call first argument must be a function but is an unresolved reference to time.Internalftl, does is need to be exported?
call first argument must be a function in an ftl module, does it need to be exported?

//ftl:verb
func Internal(ctx context.Context, req TimeRequest) (TimeResponse, error) {
	return TimeResponse{Time: time.Now()}, nil
}

[jonathanj-square]

covered by https://github.com/jonathanj-square