Compiler error message for endpoint objects in sequence local

[bmistree]

We disallow passing references to endpoints in sequence local variables. This is because an endpoint object is fundamentally a reference to an endpoint (ie, it can't be made into a value type) and it does not seem like a good idea to leak references across the network (what happens if connectivity between the host holding a reference and the host hosting the data that the reference points to goes down?).

However, the compiler doesn't actually throw a syntax error when a program using an endpoint as a sequence local variable is generated. Instead it asserts out during runtime with a very cryptic error message. Should fix.