Current BuildConfigFromFlags() seems to print a log like below when
config path isn't specified:
```
W0120 15:30:06.196820 13323 client_config.go:359] error creating
inClusterConfig, falling back to default config: %vunable to load
in-cluster configuration, KUBERNETES_SERVICE_HOST and
KUBERNETES_SERVICE_PORT must be defined
```
This commit removes the needless "%v"
For UX, it would be better if we presented an error for validation that
is "this config is empty" rather than the inaccurate "no server name" or
"no context name" errors. Returns a typed error.
