Provide a string literal-valued alternative to the final switch case.

This allows the runtime simply to set context.next = "end" in order to
jump to the final case.
Dispatch exceptions to the "end" location by default.

This ensures that exceptions will be thrown even if we don't encounter an
enclosing catch block.
Add a test of try-finally blocks with uncaught exceptions.

Existing tests of try-finally constructs all had return statements in
their finally blocks, which override any pending exception, so the actual
throwing of uncaught exceptions was not being tested.
Add a failing test of exceptional delegated yield* expressions.
Catch and dispatch exceptions thrown by context.delegate.{next,throw}.
Merge pull request #50 from facebook/exceptional-delegated-yield

Handle uncaught exceptions from delegated yield*s.