Help message on invalid component

Passing an invalid component name prints a friendly help message
instead of crashing with a stack trace. This is now consistent with a
situation when an invalid switch is passed.

lib/cli.ex

@@ -10,10 +10,12 @@ defmodule Onigumo.CLI do
            strict: [working_dir: :string]
          ) do
       {switches, [component], []} ->
-        {:ok, module} = Map.fetch(@components, String.to_atom(component))
-        working_dir = Keyword.get(switches, :working_dir, File.cwd!())
-        module.main(working_dir)
-
+        with {:ok, module} <- Map.fetch(@components, String.to_atom(component)) do
+          working_dir = Keyword.get(switches, :working_dir, File.cwd!())
+          module.main(working_dir)
+        else
+          :error -> usage_message()
+        end
       _ ->
         usage_message()
     end

test/onigumo_cli_test.exs

@@ -21,7 +21,7 @@ defmodule OnigumoCLITest do
   describe("Onigumo.CLI.main/1") do
     for argument <- @invalid_arguments do
       test("run CLI with invalid argument #{inspect(argument)}") do
-        assert_raise(MatchError, fn -> Onigumo.CLI.main([unquote(argument)]) end)
+        assert usage_message_printed?(fn -> Onigumo.CLI.main([unquote(argument)]) end)
       end
     end