add DriverErr to retrieve the original database error

[mna]

Hi Russ,

I really like that package, I feel it's just the right (thin) set of features above Go's database package. The only thing I felt was missing was a way to retrieve the original driver's error, as it was hidden away by the fmt.Errorf calls. It can be important to determine what caused the error (e.g. duplicate row, constraint error, etc.). So I added an internal error type that wraps the original error and preserves the same error messages as before, but with a DriverErr function to be able to retrieve that original error.

There's also a test for that new feature.

Thanks,
Martin