Fix validate twice

[nezuo]

The data is already validated in Document:readyPromise().

[evaera]

If we're going to remove one, I think we should remove the one in readyPromise. This feels like the one that would be reused more, so we should make sure it's always validated here.

[nezuo]

The reason I kept the one in readyPromise was because it has some custom error logic as apposed to just an assert. Should I move that error to the other location?

[evaera]

Can you make the error in DocumentData an if statement instead of an assert, and then error() with

error(Error.new({
					kind = Error.Kind.SchemaValidationFailed,
					error = schemaError,
					context = ("Schema validation failed when loading data in collection %q key %q"):format(
						self.collection.name,
						self.name
					)
				}))