PYTHON-2631 Add missing error message to InvalidBSON error (#589)

(cherry picked from commit cc029a1)

Author: ShaneHarvey

bson/_cbsonmodule.c

@@ -2621,7 +2621,7 @@ static int _element_to_dict(PyObject* self, const char* string,
     if (name_length > BSON_MAX_SIZE || position + name_length >= max) {
         PyObject* InvalidBSON = _error("InvalidBSON");
         if (InvalidBSON) {
-            PyErr_SetNone(InvalidBSON);
+            PyErr_SetString(InvalidBSON, "field name too large");
             Py_DECREF(InvalidBSON);
         }
         return -1;

test/test_bson.py

@@ -373,6 +373,13 @@ def test_invalid_decodes(self):
                 with self.assertRaises(InvalidBSON, msg=msg):
                     list(decode_file_iter(scratch))
 
+    def test_invalid_field_name(self):
+        # Decode a truncated field
+        with self.assertRaises(InvalidBSON) as ctx:
+            decode(b'\x0b\x00\x00\x00\x02field\x00')
+        # Assert that the InvalidBSON error message is not empty.
+        self.assertTrue(str(ctx.exception))
+
     def test_data_timestamp(self):
         self.assertEqual({"test": Timestamp(4, 20)},
                          decode(b"\x13\x00\x00\x00\x11\x74\x65\x73\x74\x00\x14"