Implemented first PoC

FYI. We started to use a separate error file for convenience. See https://github.com/giantswarm/tokend/blob/b263edebda501f46e845c7fa5f778814123aecfe/service/error.go

So you just do e.g. `return nil , Mask(err)`
- that removes the errgo import from all files inside a package
- that enables you to pre define errors
- that makes a line even a bit shorter