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/routes.go

@@ -59,6 +59,7 @@ func (s *Server) handlers() handlers {
 
 func (s *Server) RegisterRoutes() http.Handler {
 	e := echo.New()
+	e.HTTPErrorHandler = httpErrorHandler
 	if len(s.cfg.TrustedProxies) > 0 {
 		var trust []echo.TrustOption
 		for _, ipNet := range s.cfg.TrustedProxies {