Terminal continuation error handling

[aztecrex]

Errors thrown from continuations passed to terminal bindings such as forEach(..) and on(..) are silently dropped right now. There should be a mechanism by which such errors can be handled by the programmer.

The programmer is free, of course, to put error handling within the continuation function but this may be counter to expressiveness for unexpected conditions such as out of memory.

[aztecrex]

Would be great if this could be scoped. Some ideas:

1. install a terminal error handler at Async construction
2. install a terminal error handler in of(..) and broken(..)
3. install a scope at Async construction, scope designates error handling
4. install a scope in of(..) and broken(..), scope designates error handling

A primary objective is to keep it simple.

The above ideas could get pretty complex if a promise could have multiple handlers or
scopes. Those could possibly be mitigated by a scoped promise factory but I cringe even
writing those words together.