Return 404 Not Found responses as JSON

Signed-off-by: Christian Haudum <christian.haudum@gmail.com>

docs/sources/upgrading/_index.md

@@ -49,8 +49,6 @@ The response body has the following schema:
 }
 ```
 
-"404 Not Found" errors still return responses with content type `text/plain`.
-
 ### Promtail
 
 #### `gcplog` labels have changed

pkg/loki/loki.go

@@ -304,6 +304,7 @@ func (t *Loki) Run(opts RunOpts) error {
 
 	t.serviceMap = serviceMap
 	t.Server.HTTP.Path("/services").Methods("GET").Handler(http.HandlerFunc(t.servicesHandler))
+	t.Server.HTTP.NotFoundHandler = http.HandlerFunc(serverutil.NotFoundHandler)
 
 	// get all services, create service manager and tell it to start
 	var servs []services.Service

pkg/util/server/error.go

@@ -34,9 +34,13 @@ type ErrorResponseBody struct {
 	Message string `json:"message"`
 }
 
+func NotFoundHandler(w http.ResponseWriter, r *http.Request) {
+	JSONError(w, 404, "page not found")
+}
+
 func JSONError(w http.ResponseWriter, code int, message string, args ...interface{}) {
-	w.WriteHeader(code)
 	w.Header().Set("Content-Type", "application/json; charset=utf-8")
+	w.WriteHeader(code)
 	json.NewEncoder(w).Encode(ErrorResponseBody{
 		Code:    code,
 		Status:  "error",