add detail on parse error location and cause

This requires https://github.com/bos/attoparsec/issues/71 to have any impact, but it should work as well before with earlier attoparsecs.
I'm not sure I understand -- do you mean I should drop the call to eitherResult entirely?

The point of thie change is that (given bos/attoparsec#71 is merged) the `<?>`-comments will actually be shown.  Without this change, they will be silently dropped.  I've added another commit message to the attoparsec patch that explains this.