Litmus fail does not interpolate arguments

Litmus#fail is currently implemented with String.format which does not accept a pair of curly brackets '{}' for formatting variables.
For example,

litmus.fail("msg is {}", "some error");
=> "msg is {}"

I see 2 options:

1. Change every call to fail that uses {}
2. fail should use the sl4j MessageFormatter

I'm going to open a PR with strategy 2. It's nice to keep the same message format as the logger, otherwise it's easy to introduce bugs there. There are currently only 2 places that use the "%s" format to call fail, so those can be easily changed.