Return every error as {"error": "..."}

Echo errors, middleware and handlers used three different formats (message object, error object, bare string). One error handler now writes all of them the same way; status codes are unchanged.

internal/server/sync_handler.go

@@ -57,7 +57,7 @@ func (s *SyncHandler) SyncSoundtracksNewFull(ctx *echo.Context) error {
 
 func (s *SyncHandler) start(ctx *echo.Context, full bool, message string) error {
 	if !s.sync.Start(full) {
-		return ctx.JSON(http.StatusLocked, "Syncing is in progress")
+		return echo.NewHTTPError(http.StatusLocked, "Syncing is in progress")
 	}
 	logging.GetLogger().Info(message)
 	return ctx.JSON(http.StatusOK, message)