Add support for flatMapErr

[betamatt]

Useful for handling errors when there is a known fallback for some or all failure cases. Languages like Swift (flatMapErr) and Rust (or_else) support this in their Result types.

Ex:

result = some_operation();
result.flatMapErr(err -> 
  switch (err) {
    case SOFT_ERROR:
      return result_of_fallback_operation();
   case HARD_ERROR:
      return Result.err(err);
  }

Replaces #26