[error] Align interface with official implementation golang/xerrors

[at15]

https://github.com/golang/xerrors

This repository holds the transition packages for the new Go 1.13 error values

gommon/errors should keep same interface so migrating to standard library is easier

• [x] handle nil input https://github.com/golang/go/issues/30970
• [x] support As

[at15]

It is already in dev tree, also it is using reflectlite (which is internal) https://github.com/golang/go/tree/master/src/errors I am not sure if gommon/errors is still needed in that case ....

[at15]

Fixed in #119