Expose Lib-specific Error Ctor

[harrysolovay]

Thoughts on exposing a lib-specific error? Would allow us to catch SCALE-specific errors with an instanceof check.

import * as $ from "https://deno.land/x/scale/mod.ts";

try {
  $.object(
    ["name", $.str],
    ["nickName", $.str],
    ["superPower", $.option($.str)],
  ).encode(1618 as any);
} catch(e) {
  if (e instanceof $.ScaleError) {
    e;
  }
}

Not sure what fields we may want to attach to e (if any). It'd be nice if we could attach state/cues to help debugging.