Issue a better error message when someone decides to meddle with the active transaction from within a context manager.

lib/sqlalchemy/orm/session.py

@@ -244,9 +244,9 @@ class SessionTransaction(object):
                 "The transaction is inactive due to a rollback in a "
                 "subtransaction.  Issue rollback() to cancel the transaction.")
 
-    def _assert_is_open(self):
+    def _assert_is_open(self, error_msg="The transaction is closed"):
         if self.session is None:
-            raise sa_exc.InvalidRequestError("The transaction is closed")
+            raise sa_exc.InvalidRequestError(error_msg)
 
     @property
     def _is_transaction_boundary(self):
@@ -438,6 +438,7 @@ class SessionTransaction(object):
         return self
 
     def __exit__(self, type, value, traceback):
+        self._assert_is_open("Cannot end transaction context. The transaction was closed from within the context")
         if self.session.transaction is None:
             return
         if type is None: