RecordSet validation is not really informative

Writing
```
t.add_resource(RecordSet(
HostedZoneId="somezoneid",
Name="somename",
TTL="60",
Type="CNAME"))
```
gives me the following error:

```
print(t.to_json())
File "/usr/local/lib/python3.6/site-packages/troposphere/__init__.py", line 649, in to_json
sort_keys=sort_keys, separators=separators)
File "/usr/local/Cellar/python/3.6.5_1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/json/__init__.py", line 238, in dumps
**kw).encode(obj)
File "/usr/local/Cellar/python/3.6.5_1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/json/encoder.py", line 201, in encode
chunks = list(chunks)
File "/usr/local/Cellar/python/3.6.5_1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/json/encoder.py", line 430, in _iterencode
yield from _iterencode_dict(o, _current_indent_level)
File "/usr/local/Cellar/python/3.6.5_1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/json/encoder.py", line 404, in _iterencode_dict
yield from chunks
File "/usr/local/Cellar/python/3.6.5_1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/json/encoder.py", line 353, in _iterencode_dict
items = sorted(dct.items(), key=lambda kv: kv[0])
TypeError: '<' not supported between instances of 'NoneType' and 'str'
```

By trial and error I figured it expects ```RecordSet("some string", HostedZoneId...``` to be provided. I think it would be better with informative error message.