Do not assume all load path exist

A path can be added to the load path and
then removed from the filesystem.

Signed-off-by: José Valim <jose.valim@plataformatec.com.br>

Author: José Valim

lib/mix/lib/mix/task.ex

@@ -68,13 +68,19 @@ defmodule Mix.Task do
     # entire load path so make sure we only return unique modules.
 
     for(dir <- dirs,
-        {:ok, files} = :erl_prim_loader.list_dir(to_charlist(dir)),
-        file <- files,
+        file <- safe_list_dir(to_charlist(dir)),
         mod = task_from_path(file),
         do: mod)
     |> Enum.uniq
   end
 
+  defp safe_list_dir(path) do
+    case :erl_prim_loader.list_dir(path) do
+      {:ok, paths} -> paths
+      {:error, _} -> []
+    end
+  end
+
   @prefix_size byte_size("Elixir.Mix.Tasks.")
   @suffix_size byte_size(".beam")