Proper error message for bad args to 'mix help'

Closes #2179.

Author: ericmj

lib/mix/lib/mix/tasks/help.ex

@@ -53,6 +53,10 @@ defmodule Mix.Tasks.Help do
     shell.info "Location: #{where_is_file(module)}"
   end
 
+  def run(_) do
+    raise Mix.Error, message: "Unexpected arguments, expected `mix help` or `mix help TASK`"
+  end
+
   defp format_task(task, max, doc) do
     String.ljust(task, max) <> " # " <> doc
   end

lib/mix/test/mix/tasks/help_test.exs

@@ -39,4 +39,10 @@ defmodule Mix.Tasks.HelpTest do
       assert output =~ ~r/^Location:/m
     end
   end
+
+  test "bad arguments" do
+    assert_raise Mix.Error, "Unexpected arguments, expected `mix help` or `mix help TASK`", fn ->
+      Mix.Tasks.Help.run ["foo", "bar"]
+    end
+  end
 end