Sometimes, you just need to signal failure, and the logic doesn't make sense to encapsulate within `.to.be.not` type chaining. This commit adds an explicit `fail()` assertion which forces failure. Since there's no context to expect() with which to construct an 'expected ... to be ...` type failure message, also let a message be specified with an optional first argument: `fail("custom message")`.
…e symlinks. Fixes #3