Improve error message

Author: José Valim

mix.exs

@@ -3,7 +3,7 @@ defmodule Mix.Tasks.Compile.Sundown do
 
  def run(_) do
  if Mix.shell.cmd("make priv/markdown.so") != 0 do
- raise Mix.Error, message: "could not compile markdown.so"
+ raise Mix.Error, message: "could not run `make priv/markdown.so`. Do you have make and gcc installed?"
  end
  end
 end