Improve API Error Rendering for Nested/Child/Relations

The following discussion from !66793 (merged) should be addressed:

• @leetickett started a discussion:

  I want to raise a follow up to look at error handling. At the moment, if we try and create a timelog with a summary > 255 characters the error looks like:

  {
      "message": {
          "timelogs": [
              "is invalid"
          ]
      }
  }

  I've been doing some messing, and it's definitely possible to get "a better" error. But I feel:

  • it's outside the scope of this MR
  • it would be best if we could introduce something generic/dynamic

  Here are my ramblings for follow up:

  
  (byebug) model.errors
  #<ActiveModel::Errors:0x0000560a99906bc8 @base=#<Issue id:18 gitlab-org/gitlab-shell#8>, @errors=[#<ActiveModel::Error attribute=timelogs, type=invalid, options={}>]>
  (byebug) model.errors.errors.each { |error| puts error.full_message }
  Timelogs is invalid
  [#<ActiveModel::Error attribute=timelogs, type=invalid, options={}>]
  (byebug) model.timelogs.last.errors.errors
  [#<ActiveModel::Error attribute=summary, type=too_long, options={:count=>255}>]
  
      def all_error_messages(model)
        model.errors.each do |column, errors|
          if model.send(:"#{column}").respond_to?(:each)
            invalids = model.send(:"#{column}").reject(&:valid?)
            invalids.each do |invalid|
              invalid.errors.each do |error|
                model.errors.add(error.attribute, error.message)
              end
            end
          elsif model.send(:"#{column}").respond_to?(:errors)
  
          end
        end
      end