Improve "failed to read ~/.kube/config" error message
Improve "failed to read ~/.kube/config" error message

Trivial change to add more context to the "error while reading
$KUBECONFIG" error message.

I was recently confused by the non-specificity of the error message
that occurs in this situation, and if _I'm_ confused, then I feel bad
for other users :/

Before:
```console
 % KUBECONFIG=/noexist ./kubecfg update
ERROR invalid configuration: no configuration has been provided
```

After:
```console
 % KUBECONFIG=/noexist kubecfg update
ERROR Unable to read kubectl config: invalid configuration: no configuration has been provided
```

ksonnet/kubecfg