DEA download error can leak credentials

Filter the credentials out of DownloadError's meessage and data bag.  Without this change, download failure exceptions can make it back to the end user with all credentials intact.

```
REQUEST: PUT https://api.10.244.0.34/v2/apps/a829b7a6-1e49-46b8-bad9-ec763af6107e
REQUEST_HEADERS:
  Authorization : [PRIVATE DATA HIDDEN]
  Content-Length : 34
  Content-Type : application/json
REQUEST_BODY: {"console":true,"state":"STARTED"}
RESPONSE: [400]
RESPONSE_HEADERS:
...
RESPONSE_BODY:
{
  "code":170001,
  "description":"Staging error: failed to stage application:\n
Error downloading: http://user:pass@10.244.0.22:9022/staging/apps/a829b7a6-1e49-46b8-bad9-ec763af6107e (Response status: unknown)",
  "error_code":"CF-StagingError",
...
```