fix non-constant format string in call to fmt.Errorf

[ameowlia]

• Read the Contributing document.

Summary

Fix this error

# code.cloudfoundry.org/gorouter/config
gorouter/config/config.go:626:22: non-constant format string in call to fmt.Errorf
gorouter/config/config.go:636:22: non-constant format string in call to fmt.Errorf
gorouter/config/config.go:646:22: non-constant format string in call to fmt.Errorf
gorouter/config/config.go:663:22: non-constant format string in call to fmt.Errorf
gorouter/config/config.go:743:23: non-constant format string in call to fmt.Errorf
gorouter/config/config.go:757:22: non-constant format string in call to fmt.Errorf
gorouter/config/config.go:768:21: non-constant format string in call to fmt.Errorf
gorouter/config/config.go:781:21: non-constant format string in call to fmt.Errorf
gorouter/config/config.go:787:21: non-constant format string in call to fmt.Errorf
gorouter/config/config.go:800:21: non-constant format string in call to fmt.Errorf
gorouter/config/config.go:813:21: non-constant format string in call to fmt.Errorf
gorouter/config/config.go:830:21: non-constant format string in call to fmt.Errorf
gorouter/config/config.go:951:27: non-constant format string in call to fmt.Errorf

Backward Compatibility

Breaking Change? No