Completely remove old non-structured errors and migrate existing ones.

spec/models/code_ocean/error_spec.rb

@@ -1,19 +0,0 @@
-require 'rails_helper'
-
-describe CodeOcean::Error do
-  let(:error) { described_class.create }
-
-  it 'validates the presence of an execution environment' do
-    expect(error.errors[:execution_environment_id]).to be_present
-  end
-
-  it 'validates the presence of a message' do
-    expect(error.errors[:message]).to be_present
-  end
-
-  describe '.nested_resource?' do
-    it 'is true' do
-      expect(described_class.nested_resource?).to be true
-    end
-  end
-end