util, lang, etcd: Move the error type to our util package

We use this error in a lot of places, let's centralize it a bit.

etcd/error.go

@@ -30,10 +30,10 @@
 package etcd
 
 import (
-	"github.com/purpleidea/mgmt/etcd/interfaces"
+	"github.com/purpleidea/mgmt/util"
 )
 
 const (
 	// errInconsistentApply means applyDeltaEvents wasn't consistent.
-	errInconsistentApply = interfaces.Error("inconsistent apply")
+	errInconsistentApply = util.Error("inconsistent apply")
 )