Error-testing be with cucumber/capybaraweb
Personally, I only write feature tests for critical and very common cases, and outsource more specific tests (i.e is a certain field on a model validated) to model specs. 

I wouldn't consider it a feature that a certain validation error is shown to the user, but I consider it a feature that the user gets informed that something went wrong and his post couldn't be created. 

TL;DR Test the user experience in a feature spec and data integrity in model / controller specs.

Use different thinking hats for different tests:


Feature tests: Think in the user's perspective, how does the user interact with your app? what is the user expecting from your app?
Model tests: Think compulsive orderliness, how can you ensure data integrity?
Controller tests: Think paranoid, who can access what? does your controller comply with your API specs? 


I would test in a feature spec for a "post":


That the user is able to create a post with an expected set of information (i.e the minimum that is required to create a post) and is redirected to the new post which shows the entered information.
That the user is unable to create a post by just hitting the "Submit" button without filling anything in and is shown a validation error. After correcting the user's mistake he should be able to create the post and be redirecred to the new post which shows the entered information.


This ensures the following "features":


Posts can be created
The user is informed about mistakes
Mistakes can be fixed
The user is assured that his post has been created.


I would test in a model spec:

Spec.describe Post, type: :model do
  subject { create(:post) }

  context 'validations' do
    it { should validate_presence_of(:title) }
    # for my taste, it's good enough to test that we validate the length
    it { should validate_length_of(:title) }

    it { should validate_presence_of(:body) }
    # if you want to be super-strict, you can check the length
    it { should validate_length_of(:body).is_at_least(100) }
  end
end