Add error-handling exercise

[ageron]

This draft exercise covers various error handling techniques:

• Result.try and the try operator ?
• Result.mapErr
• Using machine-friendly errors (i.e., meaningful tag + relevant payload)
• Where/when to produce error messages
• Error message internationalization

It does not cover:

• Handling errors when using effects (Task)
• Mixing Result and Task
• Releasing resources in case of error

I think we have two options to cover the remaining points:

• Complete this exercise, e.g., by reading the user database from a file (e.g., a JSON file?)
• Create a separate exercise dedicated to these points

[Anton-4]

This looks really good so far @ageron, a well chosen example :)

Create a separate exercise dedicated to these points

I think that's best, we don't want to introduce too many new things at once.

[Anton-4]

Thanks @ageron!

[isaacvando]

Love it!