return Auth_OpenID_FailureResponse instead null when curl post or get fails due to certificate errors

I suggest:

to let consumer::begin return ```Auth_OpenID_FailureResponse``` object with a meaningful error message text instead simply ```null``` when the underlying curl post or get fails due to certificate errors.

#### Purpose:

to signal when the ```curl post``` or ```curl get``` (in Yadis ParanoidHTTPFetcher.php) does not return a request object. For example, if Host certificate verification fails due to self-signed or untrusted CAs.

The current php-openid method Consumer::begin returns only a simple "null" upstream, my patch now returns the Auth_OpenID_FailureResponse with a message.

See also some lines below the patch: beginWithoutDiscovery already returns such an object in a similar way 

```return new Auth_OpenID_FailureResponse(null,
              "OpenID 1 requests MUST include the identifier " .
              "in the request.");
```

Patch works for me, but should be further tested in different environments to make sure that it does not break other code.