api/render: use default error messages if none specified

api/render/render.go

@@ -116,11 +116,17 @@ func Error(w http.ResponseWriter, err error) {
 
 // BadRequest renders the JSON representation of err into w and sets its
 // status code to http.StatusBadRequest.
+//
+// In case err is nil, a default error message will be used in its place.
 func BadRequest(w http.ResponseWriter, err error) {
 	codedError(w, http.StatusBadRequest, err)
 }
 
 func codedError(w http.ResponseWriter, code int, err error) {
+	if err == nil {
+		err = errors.New(http.StatusText(code))
+	}
+
 	var wrapper = struct {
 		Status  int    `json:"status"`
 		Message string `json:"message"`

api/render/render_test.go

@@ -69,6 +69,11 @@ func TestErrors(t *testing.T) {
 			code: http.StatusBadRequest,
 			body: `{"status":400,"message":"assert.AnError general error for testing"}`,
 		},
+		1: {
+			fn:   BadRequest,
+			code: http.StatusBadRequest,
+			body: `{"status":400,"message":"Bad Request"}`,
+		},
 	}
 
 	for caseIndex := range cases {