refactor: centralize HTTP error handling (#2062)

This changes the error handling to a centralized HTTP error handler in `pkg/routes/error_handler.go` that converts all error types to proper HTTP responses. This simplifies the overall error handling because http handler now only need to return the error instead of calling HandleHTTPError as previously.
It also removes the duplication between handling errors with and without Sentry.

🐰 Hop along, dear errors, no more wrapping today!
We've centralized handlers in a shiny new way,
From scattered to unified, the code flows so clean,
ValidationHTTPError marshals JSON supreme!
Direct propagation hops forward with glee,
A refactor so grand—what a sight to see! 🎉

pkg/webtests/user_password_request_token_test.go

@@ -22,7 +22,6 @@ import (
 
 	apiv1 "code.vikunja.io/api/pkg/routes/api/v1"
 	"code.vikunja.io/api/pkg/user"
-	"github.com/labstack/echo/v4"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -41,7 +40,7 @@ func TestUserRequestResetPasswordToken(t *testing.T) {
 	t.Run("Invalid email address", func(t *testing.T) {
 		_, err := newTestRequest(t, http.MethodPost, apiv1.UserRequestResetPasswordToken, `{"email": "user1example.com"}`, nil, nil)
 		require.Error(t, err)
-		assert.Equal(t, http.StatusBadRequest, err.(*echo.HTTPError).Code)
+		assert.Equal(t, http.StatusBadRequest, getHTTPErrorCode(err))
 	})
 	t.Run("No user with that email address", func(t *testing.T) {
 		_, err := newTestRequest(t, http.MethodPost, apiv1.UserRequestResetPasswordToken, `{"email": "user1000@example.com"}`, nil, nil)