Copy number variation segment files of PRAD cannot be downloaded

- Those files can be searched on GDC data portal.
- But, when using API to access those files, return
```
{
"message":"internal server error"
}
```
Right Now:
Choose to avoid this error by skipping downloading the CNV segment files for PRAD.
```
if "message" in json.loads(response.content.decode("utf-8")).keys():
    return None, 'Not found'
```