concurrent.futures: Add various Errors. Fix wait(). (#964)

Introduces Error, CancelledError and TimeoutError, based on the backport.
Also fixes the return type of wait to be Sets instead of Iterables. The
function documentation promises Sets and the code uses Sets.

https://github.com/agronholm/pythonfutures/blob/master/concurrent/futures/_base.py#L261

third_party/2/concurrent/futures/__init__.pyi

@@ -1,7 +1,11 @@
-from typing import TypeVar, Generic, Any, Iterable, Iterator, Callable, Tuple, Union
+from typing import TypeVar, Generic, Any, Iterable, Iterator, Callable, Optional, Set, Tuple, Union
 
 _T = TypeVar('_T')
 
+class Error(Exception): ...
+class CancelledError(Error): ...
+class TimeoutError(Error): ...
+
 class Future(Generic[_T]):
     def cancel(self) -> bool: ...
     def cancelled(self) -> bool: ...
@@ -28,7 +32,7 @@ class ThreadPoolExecutor(Executor):
 class ProcessPoolExecutor(Executor):
     def __init__(self, max_workers: Union[int, None] = ...) -> None: ...
 
-def wait(fs: Iterable[Future], timeout: float = ..., return_when: str = ...) -> Tuple[Iterable[Future], Iterable[Future]]: ...
+def wait(fs: Iterable[Future], timeout: Optional[float] = ..., return_when: str = ...) -> Tuple[Set[Future], Set[Future]]: ...
 
 FIRST_COMPLETED = ...  # type: str
 FIRST_EXCEPTION = ...  # type: str