apt_key clear message when downloading key returns an (http) error code

When downloading the key fails it currently gives a very non descriptive error.

In case of a 403 Forbidden for example Ansible returns:
```
failed: [1234.trbs.net] => {"failed": true, "traceback": "Traceback (most recent call last):\n  File \"/root/.ansible/tmp/ansible-tmp-1403701692.1-118748049158066/apt_key\", line 158, in download_key\n    return rsp.read()\nAttributeError: 'NoneType' object has no attribute 'read'\n"}
msg: error getting key id from url: http://apt.trbs.com/forbidden.key

FATAL: all hosts have already failed -- aborting
```

This patch uses the error message from the ```fetch_url``` function to give a useful message:
```
failed: [1234.trbs.net] => {"failed": true}
msg: Failed to download key at http://apt.trbs.com/forbidden.key: HTTP Error 403: Forbidden

FATAL: all hosts have already failed -- aborting
```