Refactor impls in result.rs

[BoySanic]

          I kind of dislike having so many impls here, I might refactor it later.

Originally posted by @DutChen18 in #53 (comment)

[DutChen18]

After taking another look I'm not actually so sure there's a better way.

[DutChen18]

There is currently no good way inside these impls to generate an error for RowNotFound specific to the table. So looks like I'll have to refactor this anyways.