pkg/errors dependency

[laoshaw]

pkg/errors is no longer maintained, cockroachdb/errors still depends on it, will that be an issue down the road.

looking for a pkg/errors replacement and just found this project, will start using it.

[knz]

thanks!

the dependency is there just to reuse a particular data type there. Even if it's not maintained any more, that part still works. We can drop the dependency probably by re-implementing the same type.