guru: add :GoWhicherrs

[krishicks]

Hi,

Alan Donovan at Gophercon 2016 showed that guru whicherrs is a thing, which I noticed was missing from vim-go. This is my attempt at adding it.

Cheers,

KH

[fatih]

Thanks @krishicks 👍

There are couple improvements we need to make (such as displaying and handling the case where no errs are available and it results in empty output). I'll push those myself.

[krishicks]

Thanks!