[3.5] Clarify exception handler scope in contextlib (GH-1104)

Moved explicit raise from inside try to try...else.
(cherry picked from commit 680e20b)

Author: Mariatta

Lib/contextlib.py

@@ -75,7 +75,6 @@ def __exit__(self, type, value, traceback):
                 value = type()
             try:
                 self.gen.throw(type, value, traceback)
-                raise RuntimeError("generator didn't stop after throw()")
             except StopIteration as exc:
                 # Suppress StopIteration *unless* it's the same exception that
                 # was passed to throw().  This prevents a StopIteration
@@ -101,6 +100,8 @@ def __exit__(self, type, value, traceback):
                 #
                 if sys.exc_info()[1] is not value:
                     raise
+            else:
+                raise RuntimeError("generator didn't stop after throw()")
 
 
 def contextmanager(func):