Make credentialprovider less verbose about benign errors.

In particular, a few of the utilities used within the credentialprovider had the pattern:
```
    glog.Errorf("while blah %s: %v", s, err)
    return nil, err
```

This change composes the error message that is propagated and puts the burden of logging on the caller, which gives us the new pattern:
```
    return nil, fmt.Errorf("while blah %s: %v", s, err)
```

In particular, this allows us to squelch all output during kubelet startup when we are detecting whether certain credentialprovider plugins should even be enabled.

Fixes: https://github.com/GoogleCloudPlatform/kubernetes/issues/2673
Implementing the error was quick enough, so I just did it.

LMK if you have any other feedback.