Fix error message from badly formatted duration

This fixes the particular error message that is reported when
trying to parse a time interval string that's badly formatted.

At the moment, when trying to parse something like:
```
PT5D
```
it reports:
```
isodatetime.parsers.ISO8601SyntaxError: Invalid ISO 8601 date representation: 
```
whereas it now reports:
```
isodatetime.parsers.ISO8601SyntaxError: Invalid ISO 8601 duration representation: PT5D
```

@matthewrmshin, please review.