adds support of explict argument for #rule_error? method

[moofkit]

Closing #658

Adds support for #rule_error? for checking that other rule has error

Example

class PersonContract < Dry::Validation::Contract
  schema do
    required(:email).filled(:string)
    required(:name).filled(:string)
  end

  rule(:name) do
    key.failure('name rule error')
  end

  rule(:email) do
    key.failure('email rule error') if rule_error?(:name)
  end
end

PersonContract.new.call(email: 'bar', name: 'foo').errors.to_h
# {name: ['name rule error'], email: ['email rule error']}

[solnic]

This checks all errors, not just rule errors, so I'm afraid this doesn't do what we want. One option to make this work would be to add Result#rule_error? that would filter out schema errors and look only at rule errors.

[moofkit]

The problem here that schema and rule errors are mixed in Result#errors set.
So I see two possible solutions:

1. Filter schema errors keys from errors set and searching in that subset.
2. Check that there are no schema error with provided key and then look up Result#errors.

The first approach seems to me is a little more straightforward.
Because in second one you need to know that rule dependent on schema and does not evaluate if there are some error.
But the second one was easier to implement 😄

If second approach is not ok for you I will rewrite it.

[moofkit]

@solnic thanks for your review!

I have fixed issue. Can you please have a look one more time? Or I need to rewrite it with another approach?

[solnic]

@moofkit hey sorry for the delay, I was on a sick leave. Thanks for working on this PR. As you can see I just merged it and it'll be included in the 1.6.0 release.

[moofkit]

@solnic thanks! Hope your are well for now!