PYTHON-2438 Fix str representation of BulkWriteError (#522)

(cherry picked from commit 86d5811)

Author: pcorpet

pymongo/errors.py

@@ -240,8 +240,9 @@ class BulkWriteError(OperationFailure):
     def __init__(self, results):
         super(BulkWriteError, self).__init__(
             "batch op errors occurred", 65, results)
-        # For pickle support
-        self.args = (results,)
+
+    def __reduce__(self):
+        return self.__class__, (self.details,)
 
 
 class InvalidOperation(PyMongoError):

test/test_errors.py

@@ -93,6 +93,7 @@ def test_pickle_OperationFailure(self):
     def test_pickle_BulkWriteError(self):
         exc = BulkWriteError({})
         self.assertOperationFailureEqual(exc, pickle.loads(pickle.dumps(exc)))
+        self.assertIn("batch op errors occurred", str(exc))
 
     def test_pickle_EncryptionError(self):
         cause = OperationFailure('error', code=5, details={},