Errors that occour in a ClojureScript pREPL connection are :ret as data under :val, not as a string

I'm working on some pREPL based tooling and noticed that if a JVM Clojure pREPL throws an error it is sent to stderr as well as a `:ret` under the `:val` keyword as a string.

When ClojureScript throws an error (in this case within nodejs) there is no stderr output and the `:val` contains actual EDN data, not a string. This inconsistency breaks the assumption that `:val` will always be a string of EDN data.

I've never raised an issue with Clojure before or submitted a patch, I am happy to do so if this is deemed a valid issue and worth fixing.