add conditional error combinator

[mattfenwick]

Goal: fail to an error based on checking a parsed value using a predicate.

Something like:

// (a -> Bool) -> Parser e s (m t) a -> e -> Parser e s (m t) a
function checkError(predicate, parser, errorValue) {
  checkFunction('checkError', predicate);
  checkParser('checkError', parser);
  return bind(parser, (value) => predicate(value) ? pure(value) : error(errorValue));
  // or:
  return bind(parser, (value) => (predicate(value) ? pure : error)(value));
}

Needs corner cases to be hammered out and a better name, of course.

Cases:

• parser fails -> fail
• parser errors -> error
• parser succeeds
  • predicate passes -> success
  • predicate fails -> error

Another implementation possibility, which doesn't work because it doesn't pass the test case for "parser fails -> fail":

// (a -> Bool) -> Parser e s (m t) a -> e -> Parser e s (m t) a
function checkError(predicate, parser, e) {
    return alt([check(predicate, parser), error(e)]);
}