PYTHON-2438 Fix str representation of BulkWriteError

Author: pcorpet

pymongo/errors.py

@@ -243,6 +243,10 @@ def __init__(self, results):
         # For pickle support
         self.args = (results,)
 
+    if sys.version_info[0] != 2:
+        def __str__(self):
+            return str(self._message)
+
 
 class InvalidOperation(PyMongoError):
     """Raised when a client attempts to perform an invalid operation."""

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={},