Raise (Instead of Warn) on Misconfiguration

[rringler]

Right now, if a gem user misconfigures their excluded_fields or users file paths, the library will log a warning and then proceed as if the file was empty, which could lead to run-time errors.

It would probably be better if IronBank::Configuration#excluded_fields & IronBank::User.store raised immediately if the the config specifies a file that does not exist.