add `.original` property to custom Error classes

The current `.parent` property is a bit of a misnomer. `.original`
better conveys the intent of the field and function of the class, which
is to wrap the original database error.

This commit does not remove the current `.parent` property for backwards
compatibility.
Merge pull request #2761 from defunctzombie/original-error

add `.original` property to custom Error classes