Add basic error overriding

[Marsup]

Hey y'all,

Had some free time this week-end, I've been toying in my head with this kind of API for while now, it allows you to override errors. This PR is essentially a reality check to see if it can be useful to some people.

Now, with that said, you asked for it, but here be dragons ! You'll be dealing with the full stack of errors joi is usually manipulating for you to give you those so called ugly errors, you'll have to work a bit to make your own messages, so don't complain if it's too complex (unless you have better ideas), there's no free lunch :)

Roles are now reversed, I'll let you do the review and see where we go from here.

[Marsup]

Well, so much silence... I'll merge anyway.

[mattapperson]

When using this it seems to break the abortEarly option? error returned by validate is just the one error when using this vs being added within err.details

[mattapperson]

Edit: This is broken when validating an object of keys, it fails as if validating just like a string for example

[Marsup]

Mind providing an example ?

[mattapperson]

Sure! So with the following code, not using .error()

Joi.validate(values, {
    name: Joi.string().required().max(10),
    confirmName: Joi.string().required().valid(Joi.ref('name'))
}, {
     abortEarly: false
}, (err, value) => {
     console.log(JSON.stringify(err, false, 2))
})

I get the following in the error of the callback:

{
  ...
  "isJoi": true,
  "name": "ValidationError",
  "details": [
    {
      "message": "\"name\" length must be less than or equal to 10 characters long",
      "path": "name",
      "type": "string.max",
      "context": {
        "limit": 10,
        "value": "matthghgjhg",
        "key": "name"
      }
    },
    {
      "message": "\"confirmName\" is required",
      "path": "confirmName",
      "type": "any.required",
      "context": {
        "key": "confirmName"
      }
    }
  ],
  "_object": {
    "name": "matthghgjhg"
  }
}

however if I use the following:

Joi.validate(values, {
    name: Joi.string().required().max(10),
    confirmName: Joi.string().required().valid(Joi.ref('name')).error(new Error('Was REALLY expecting this to match your name'))
}, {
     abortEarly: false
}, (err, value) => {
     console.log(JSON.stringify(err, false, 2))
})

Then I simply get the js error "Was REALLY expecting this to match your name" returned without the "isJoi":, "name" or "details" keys, and the error is just for the confirmName field

[Marsup]

That's unrelated to this PR and is working as documented. An actual error will be served as is.

[mattapperson]

@Marsup so it is expected to not be able to get all errors from an object if one field has a custom error?

[Marsup]

When using an actual Error, yes.

[mattapperson]

Hmmm, would you be open to considering a PR to change that? Or else would you mind explaining why this should be as it is because to me this felt very unexpected. I would only expect that if abortEarly was set to true.

[Marsup]

Not really. Because it's inconsistent and often impossible in several scenarios to combine several types of errors. It's mostly used to throw boom errors anyway. If you don't want that just use the other forms.

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