Round out logging interface with functions for all levels

pkg/log/logger.go

@@ -6,10 +6,13 @@ package log
 // Logger serves as an adapter interface for logger libraries
 // so that dex does not depend on any of them directly.
 type Logger interface {
+	Debug(args ...interface{})
 	Info(args ...interface{})
 	Warn(args ...interface{})
+	Error(args ...interface{})
 
 	Debugf(format string, args ...interface{})
 	Infof(format string, args ...interface{})
+	Warnf(format string, args ...interface{})
 	Errorf(format string, args ...interface{})
 }