It's easy to put accidently put something that is not a valid tag in a
ControlSignature or ExecutionSignature. Previously, when you did that
you got a weird error at the end of a very long template instantiation
chain that made it difficult to find the offending worklet.

This adds some type checks when the dispatcher is instantated to check
the signatures. It doesn't point directly to the signature or its
parameter, but it is much closer.