The :validate option currently takes a validation function and a string error message.
Could we extend the second argument to a function instead so you can return an error message like "--foo should be greater than 10, but was: 11.
We could even plug clojure.spec.alpha or malli in there:
(def cli-options
[["-n" "--number NUM" "A number"
:parse-fn (m/decoder int? mt/string-transformer)
:validate [(fn [input]
(m/validate [:< 100] input))
(fn [input]
(me/humanize (m/explain [:< 100] input)))]]])
