Raise in Time.add/3 for non-positive integer

[03juan]

Before this fix the test fails because the code runs through System.convert_time_unit/3 into System.normalize_time_unit/1.

  1) test add/2 (TimeTest)
     lib/elixir/test/elixir/calendar/time_test.exs:94
     Wrong message for ArgumentError
     expected:
       ~r/Expected :hour, :minute, :second/
     actual:
       "unsupported time unit. Expected :second, :millisecond, :microsecond, :nanosecond, or a positive integer, got 0"
     code: assert_raise ArgumentError, ~r/Expected :hour, :minute, :second/, fn ->
     stacktrace:
       lib/elixir/test/elixir/calendar/time_test.exs:99: (test)

[03juan]

I was busy writing a code review comment on #13121 but José beat me to the commit. 🤣

[03juan]

Actions passed in my repo

[josevalim]

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