Fixed Long Standing issue with Clients causing aborts when used wrong

Summary:
Python is considered a Safe Enivronment, where things like aborts and segfaults
can not happen.

If clients where used outside of "async with" we use to print a error message
and then C++ would abort.

This fix makes sure C++ no longer aborts but we still print the message
that the "async with" should be used.

Reviewed By: ambv

Differential Revision: D6923956

fbshipit-source-id: 4f59f6c3c8aa5c8b2c279c728a8c011a040940af

folly/python/__init__.pxd

@@ -13,7 +13,7 @@ cdef extern from "folly/Try.h" namespace "folly" nogil:
 
 cdef extern from "folly/futures/Future.h" namespace "folly" nogil:
     cdef cppclass cFollyFuture "folly::Future"[T]:
-        pass
+        T get()
         #TODO add via and then
 
 cdef extern from "folly/Unit.h" namespace "folly":