Improve docs for EEx.AssignsEngine, closes #1499
mix run now properly sets System.argv
Make it explicit that some tasks require a project to be defined
test_helper files still need to be required upfront

Since they have to be required upfront, it makes no sense to have
a Code.require_file("test_helper.exs", __DIR__) in Mix projects
since the files can't run in isolation anyway.
Fix some invalid error messages from Mix