Return an exception struct from put_locale_from_session/2 for an empty session

[rubas]

Before

iex> Localize.Plug.put_locale_from_session(%{})
{:error, {Localize.UnknownLocaleError, "No locale was found in the session"}}

iex> Localize.Plug.put_locale_from_session(%{"localize_locale" => "!!!"})
{:error, %Localize.InvalidLocaleError{locale_id: "!!!"}}

Problem

• The spec says {:error, Exception.t()}, and the 1.1.0 changelog says this function now reports errors as exception structs. The empty-session branch was missed.
• A caller that handles the error with Exception.message/1 or a %{__exception__: true} match fails on this one branch.
• The ### Returns doc still describes {:error, {exception, reason}}.

After

iex> Localize.Plug.put_locale_from_session(%{})
{:error, %Localize.Plug.NoSessionLocaleError{message: "No locale was found in the session"}}

Details

Cause: the fallback clause of Localize.Plug.put_locale_from_session/2 in lib/localize/plug.ex.

Design: a small Localize.Plug.NoSessionLocaleError, defined in Localize.Plug, keeps the existing message. Localize.UnknownLocaleError, which the tuple named, has only a :locale_id field and would print "The locale nil is not known."; no exception in localize takes a free message. The ### Returns doc now says {:error, exception} and names this case, and the module is listed in the Plugs docs group.

Tests: the existing empty-session test in test/localize_web_test.exs now matches %Localize.Plug.NoSessionLocaleError{} and checks its message; it fails on main.

Checks: mix test (267 passed), mix format --check-formatted, mix compile --warnings-as-errors. mix credo --strict reports only the existing Sigil_q.InterpolationTest module-name issue.

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