http: Only log handler errors >= 500

Errors in the 4xx range are client errors, and they don't need to be
entered into the server's error logs. 4xx errors are still recorded in
the access logs at the error level.

modules/caddyhttp/server.go

@@ -152,7 +152,9 @@ func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 			if err2 == nil {
 				// user's error route handled the error response
 				// successfully, so now just log the error
-				logger.Error(errMsg, errFields...)
+				if errStatus >= 500 {
+					logger.Error(errMsg, errFields...)
+				}
 			} else {
 				// well... this is awkward
 				errFields = append([]zapcore.Field{
@@ -163,7 +165,9 @@ func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 				logger.Error("error handling handler error", errFields...)
 			}
 		} else {
-			logger.Error(errMsg, errFields...)
+			if errStatus >= 500 {
+				logger.Error(errMsg, errFields...)
+			}
 			w.WriteHeader(errStatus)
 		}
 	}