Build issues `undefined: "github.com/corestoreio/errors".NewWriteFailed`

[shift]

Unable to build. The following errors are produced.

# github.com/corestoreio/caddy-esi/esitag
../../../corestoreio/caddy-esi/esitag/data_tag.go:121:20: undefined: "github.com/corestoreio/errors".NewWriteFailed
../../../corestoreio/caddy-esi/esitag/data_tag.go:155:23: undefined: "github.com/corestoreio/errors".NewWriteFailed
../../../corestoreio/caddy-esi/esitag/data_tag.go:177:24: undefined: "github.com/corestoreio/errors".NewWriteFailed
../../../corestoreio/caddy-esi/esitag/data_tag.go:183:24: undefined: "github.com/corestoreio/errors".NewWriteFailed
../../../corestoreio/caddy-esi/esitag/data_tag.go:190:25: undefined: "github.com/corestoreio/errors".NewWriteFailed
../../../corestoreio/caddy-esi/esitag/data_tag.go:199:24: undefined: "github.com/corestoreio/errors".NewWriteFailed
../../../corestoreio/caddy-esi/esitag/data_tag.go:205:24: undefined: "github.com/corestoreio/errors".NewWriteFailed
../../../corestoreio/caddy-esi/esitag/data_tag.go:211:24: undefined: "github.com/corestoreio/errors".NewWriteFailed
../../../corestoreio/caddy-esi/esitag/data_tag.go:222:23: undefined: "github.com/corestoreio/errors".NewWriteFailed
../../../corestoreio/caddy-esi/esitag/data_tag.go:228:23: undefined: "github.com/corestoreio/errors".NewWriteFailed
../../../corestoreio/caddy-esi/esitag/data_tag.go:228:23: too many errors