Add JoinError

This is actually trickier than it looks. It's not always straightforward to pick out the arity of the executable methods.

executable.method(executable.method_name).arity

Doesn't do it. Interned tasks will not be picked up correctly, nor will _method executables.

Simply fishing for ArgumentErrors is also tricky... could do it in Executable, but you have to check to make sure the error is the result of a misjoin.

        rescue ArgumentError
          raise if previous.select {|_audit| _audit.key }.empty?
          raise JoinError(self, previous)
        end

Moreover, you'd have to check to make sure the ArgumentError is coming from the call to the method and not something internal to process itself.

May be best to fish for these top-level and try to detect... OR to begin implementing some annotation regarding what comes in and what goes out.