Allow app to start w/o ERRBIT_EMAIL_FROM

When `USE_ENV` is true, would like to be able to start the app (or execute rake tasks) without having to first provide a value for `ERRBIT_EMAIL_FROM`. If you try to do this now you get a 'NoMethodError: undefined method 'split' for nil:NilClass' error in `_load_config.rb`.

This change simply makes the initializer more tolerant of a missing `ERRBIT_EMAIL_FROM` value.