feat(client): add details to custom error

[daniellacosse]

This will allow providers to send:

{
  "error": {
    "message": "There was an error.",
    "details": "<Detailed information about the error, displayed in an alert box.>"
  }
}

[fortuna]

That was quick!

[jyyi1]

I think we can directly declare details in the constructor (like constructor(message: string, private readonly details?: string), so we don't need this assignment?

[daniellacosse]

Personally I find that particular shortcut confusing, but I can use it

[jyyi1]

Cool, thanks! That's not a big issue though.

[fortuna]

Looks great, thanks!
We should post on Reddit and update the doc once it’s released