CLN/ENH: Specify Exceptions in io.common._is_url
This is `_is_url`, it has a blanket `except` statement, I'm thinking it would be better to specify the exceptions here (caused an issue for me while editing earlier). Not sure which exceptions should go here (ValueError, TypeError?), anyone have an idea?

``` python
def _is_url(url):
     try:
        return urlparse.urlparse(url).scheme in _VALID_URLS
     except:
         return False
```
