Allow Err to return an error message

[kingsleyh]

It would be helpful if Err could return an error message:

https://github.com/mint-lang/mint-runtime/blob/master/source/Result.js

so that we could do something like this:

enum KeyPair.Error {
  KeyPairGenerationError String
} 
return new Err($KeyPair_Error_KeyPairGenerationError, "some error message")

not sure if that kind of thing is supported by enum

[gdotdesign]

It is possible for a while now 😂 closing this.