Exit codes are being reused

[plexus]

We have two places where we throw an early-exit with exit code 252. We should never do this, an exit code should unambiguously at the code that caused it.

• check if we have any other duplicates
• make sure they are all unique
• update the docs

[alysbrooks]

Looks like there's duplicates of 254, too.

Here's what I found with a quick-and-dirty ripgrep search:

src/kaocha/api.clj:84:     (throw+ {:kaocha/early-exit 254}))))
src/kaocha/plugin.clj:48:      (output/error-and-throw {:kaocha/early-exit 254} nil
src/kaocha/plugin.clj:56:        {:kaocha/early-exit 254} nil
src/kaocha/plugin.clj:68:        {:kaocha/early-exit 254} nil

src/kaocha/runner.clj:166:                                                             (throw+ {:kaocha/early-exit 252})))
src/kaocha/config.clj:31:         (throw+ {:kaocha/early-exit 252}))))
test/unit/kaocha/config_test.clj:60:    (is (= #:kaocha{:early-exit 252}

[alysbrooks]

Mostly addressed in #303. Docs just need to be wrapped up.