Add supports for Phoenix.Ecto changeset errors

[Matsa59]

Imagine we add a mark that doesn't exist, an invalid changeset will be returned.

We have to transform thoses changeset to inline it and allow user to display error using

<%= error_tag f, :field %>

Example of invalid changeset

#Ecto.Changeset<
  action: nil,
  changes: %{
    title: #Ecto.Changeset<
      action: :insert,
      changes: %{
        content: #Ecto.Changeset<
          action: :insert,
          changes: %{
            content: [
              #Ecto.Changeset<
                action: :insert,
                changes: %{
                  content: [
                    #Ecto.Changeset<
                      action: :insert,
                      changes: %{text: "Hello world"},
                      errors: [marks: {"is invalid", []}],
                      data: #ExProsemirror.Block.Text<>,
                      valid?: false
                    >
                  ]
                },
                errors: [],
                data: #ExProsemirror.Block.Paragraph<>,
                valid?: false
              >
            ]
          },
          errors: [],
          data: #ExProsemirror.Block.Doc<>,
          valid?: false
        >
      },
      errors: [],
      data: #ExProsemirror.Schema<>,
      valid?: false
    >,
    title_plain: "{\"type\":\"doc\",\"content\":[{\"type\":\"p\",\"content\":[{\"type\":\"text\",\"marks\":[{\"type\":\"em\"},{\"type\":\"strong\"},{\"type\":\"underline\"}],\"text\":\"Hello world\"}]}]}"
  },
  errors: [],
  data: #ExProsemirrorSample.Article<>,
  valid?: false
>