Propagated fix from Elixir 1.12 branch to Elixir 1.7 and get `** (UndefinedFunctionError) function :rollback.errors/0 is undefined (module :rollback is not available)` when examining changeset errors

Questions / Help
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1

I made a change in our 1.12 branch (which works fine there) and we decided to cherry-pick it back to the 1.7 branch. When I run tests I get an error when trying to assert the content of changeset.errors that says:
** (UndefinedFunctionError) function :rollback.errors/0 is undefined (module :rollback is not available)
I get the same error just trying to inspect changeset.errors (or even Map.from_struct(changeset.errors)).

Is there perhaps a :rollback module embedded in 1.12 that has to be explicitly included in 1.7? (I tried adding :rollback as an extra app with no luck.) But if that’s the case why would I never have seen an error like this in any other tests that examine errors?
(Possible hint: the errors are actually from a field that is a has_one member of the current schema, and the error has been transferred from the associated schema to the virtual field on the parent schema.)

Test:

    test "error", %{channel: channel} do
      params = %{retention_policy_max_age: "0"}

      {:error, changeset} =
        channel
        |> Channel.preload_schema([:retention_policy])
        |> Channel.update(params)

      #IO.inspect(Map.from_struct(changeset.errors))
      assert changeset.errors == [
               retention_policy_max_age:
                 {"must be greater than %{number}",
                  [validation: :number, number: 0]}
      ]
    end

Error:

  1) test retention_policy error (MyApp.ChannelTest)
     myapp/models/channel_test.exs:769
     ** (UndefinedFunctionError) function :rollback.errors/0 is undefined (module :rollback is not available)
     code: assert changeset.errors == [
     stacktrace:
       :rollback.errors()
       myapp/models/channel_test.exs:778: (test)

I inspected the changeset error from the module for RetentionPolicy, before it was added to the virtual field in Channel:

17:00:36.442 [error]MyApp.Schema.Channel.handle_retention_params/2:220 #Ecto.Changeset<action: nil, changes: %{retention_policy_max_age: 0}, errors: [retention_policy_max_age: {"must be greater than %{number}", [validation: :number, number: 0]}], data: #MyApp.Settings.Schema.RetentionPolicy<>, valid?: false>

Any ideas what is going on? Using Ecto 2.1.6 on the 1.7 branch, versus 3.7.1 with 1.12.

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