fix: :bug: generate agents that return JSON errors

[ghusse]

Linked to CU-6gmxhz

This PR introduces a new dependency forest-express which was previously implied on generated projects by forest-express-{mongodb|sequelize}.

The generated app.js file now uses the errorHandler, exposed by forest-express (see https://github.com/ForestAdmin/forest-express/pull/462), to be sure that error are correctly serialized.

Pull Request checklist:

• [x] Write an explicit title for the Pull Request, following Conventional Commits specification
• [ ] Create automatic tests
• [x] No automatic tests failures
• [x] Test manually the implemented changes
• [x] Review my own code (indentation, syntax, style, simplicity, readability)
• [x] Wonder if you can improve the existing code

[arnaudbesnier]

Task linked: CU-6gmxhz P3 - Record Update - Error is in HTML instead of JSON with mongo validator on model and custom routes

[forest-bot]

:tada: This PR is included in version 3.6.5 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: