Fail if an application can't be started, closes #869

Author: José Valim

lib/mix/lib/mix/tasks/app.start.ex

@@ -43,7 +43,7 @@ defmodule Mix.Tasks.App.Start do
       case Application.Behaviour.start(app) do
         :ok -> :ok
         { :error, reason } ->
-          Mix.shell.error "Could not start application #{app}: #{inspect reason}"
+          raise Mix.Error, message: "Could not start application #{app}: #{inspect reason}"
       end
     end
   end

lib/mix/test/mix/tasks/app.start_test.exs

@@ -13,8 +13,9 @@ defmodule Mix.Tasks.App.StartTest do
     Mix.Project.push CustomApp
 
     in_fixture "no_mixfile", fn ->
-      Mix.Tasks.App.Start.run ["--no-compile"]
-      assert_received { :mix_shell, :error,["Could not start application app_start_sample: " <> _]}
+      assert_raise Mix.Error, fn ->
+        Mix.Tasks.App.Start.run ["--no-compile"]
+      end
 
       Mix.Tasks.App.Start.run ["--no-start"]
       assert File.regular?("ebin/Elixir-A.beam")