Add self.assertNotNested() in CreateString

And also add a test case.
If you try to nest CreateString you will get a clear exception.

python/flatbuffers/builder.py

@@ -350,6 +350,8 @@ class Builder(object):
     def CreateString(self, s):
         """CreateString writes a null-terminated byte string as a vector."""
 
+        self.assertNotNested()
+
         if isinstance(s, compat.string_types):
             x = s.encode()
         elif isinstance(s, compat.binary_type):

tests/py_test.py

@@ -1130,6 +1130,13 @@ class TestExceptions(unittest.TestCase):
         assertRaises(self, lambda: b.PrependUOffsetTRelative(1),
                      flatbuffers.builder.OffsetArithmeticError)
 
+    def test_create_string_is_nested_error(self):
+        b = flatbuffers.Builder(0)
+        b.StartObject(0)
+        s = 'test1'
+        assertRaises(self, lambda: b.CreateString(s),
+                     flatbuffers.builder.ObjectIsNestedError)
+
 
 def CheckAgainstGoldDataGo():
     try: