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/middleware/sync_check.go

@@ -14,7 +14,7 @@ func SyncCheck(isSyncing func() bool) echo.MiddlewareFunc {
 		return func(c *echo.Context) error {
 			if isSyncing() {
 				logging.GetLogger().Info("Syncing is in progress - request blocked")
-				return c.JSON(http.StatusLocked, "Syncing is in progress")
+				return echo.NewHTTPError(http.StatusLocked, "Syncing is in progress")
 			}
 			return next(c)
 		}