Add RemoteError and current_process to multiprocessing (#445)

stdlib/3/multiprocessing/__init__.pyi

@@ -2,6 +2,8 @@
 
 from typing import Any, Callable, Iterable, Mapping
 
+from multiprocessing.process import current_process as current_process
+
 class Lock():
     def acquire(self, block: bool = ..., timeout: int = ...) -> None: ...
     def release(self) -> None: ...

stdlib/3/multiprocessing/managers.pyi

@@ -6,3 +6,5 @@ from typing import Any
 
 class BaseManager():
     def register(typeid: str, callable: Any = ...) -> None: ...
+
+class RemoteError(Exception): ...

stdlib/3/multiprocessing/process.pyi

@@ -0,0 +1,2 @@
+def current_process(): ...
+def active_children(): ...