Reduce error to warning

With these messages, I should reduce the error to a warning:

```
[ERROR] [WallTime: 1379029882.343308] No default provider given for capability interface 'minimal_pkg/Minimal'. 
[WARN] [WallTime: 1379029882.343547] 'minimal_pkg/Minimal' has only one provider, 'minimal_pkg/minimal', using that as the default.
```