Warn if QGL2 stub tries to return

[ahelsing]

A QGL2 stub of a QGL1 function cannot return a value. The function can generate a pulse (side effect), but it cannot return a value. See if we can discover the function is trying to return something and give the user a friendly error message.

Example: You cannot write a a stub to retrieve a value from a qubit. The stub ends up needing to say 'return' to work, but then whatever you return is what is used at runtime. Useless. (The solution is to do whatever calculation you need to generate a pulse within the stubbed function.)