Introduce an easy way to create errors from `onSubmit`

[crutchcorn]

While discussing items we want to improve for a future v2, validation around onSubmit has come up a number of times.

As such, we're going to be backporting an API coming in v2 to the current version of Form (v1):

useForm({
   // ...
   onSubmit: async ({ value, createValidationError }) => {
     try {
        await callEndpoint(value)
        navigateToOverview()
     } catch (error) {
        if (error.code === 400) {
           return createValidationError({ form: `The endpoint had an issue: ${error.message}` }) 
        }
     }
   }
})

This work needs #2128 to be merged first to avoid headaches with generics, but that should come shortly.

We want to hear from you if this API works/doesn't work for your needs and, if not, why not.

Thanks all!

[joaopslins]

Hello @crutchcorn thank you for creating this issue 🙌

Quick question: This also allows field-level validation, correct? With the correct field types.

Something like:

createValidationError({
  form: `The endpoint had an issue: ${error.message}`,
  fields: {
    name: "Field-level validation is also possible"
  },
}) 

---

Also, another point raised in another discussion brought up a good point about isSubmitSuccessful. Since onSubmit wouldn't throw in this case, isSubmitSuccessful would be marked as true (source).

Perhaps this was already going to be taken into consideration, wanted to flag it just in case.

[LeCarbonator]

Yes, that is the plan @joaopslins .

Also, isSubmitSuccessful will be false, to mirror what happens when a validation failed during the submission process.