Add the error message why an IP claim is denied

ipam/allocator.go

@@ -275,7 +275,7 @@ func (alloc *Allocator) spaceRequestDenied(sender mesh.PeerName, r address.Range
 	for i := 0; i < len(alloc.pendingClaims); {
 		claim := alloc.pendingClaims[i].(*claim)
 		if r.Contains(claim.cidr.Addr) {
-			claim.deniedBy(alloc, sender)
+			claim.deniedBy(alloc, sender, nil)
 			alloc.pendingClaims = append(alloc.pendingClaims[:i], alloc.pendingClaims[i+1:]...)
 			continue
 		}

ipam/claim.go

@@ -135,12 +135,16 @@ func (c *claim) Try(alloc *Allocator) bool {
 	return true
 }
 
-func (c *claim) deniedBy(alloc *Allocator, owner mesh.PeerName) {
+func (c *claim) deniedBy(alloc *Allocator, owner mesh.PeerName, err error) {
 	name, found := alloc.nicknames[owner]
 	if found {
 		name = " (" + name + ")"
 	}
-	c.sendResult(fmt.Errorf("address %s is owned by other peer %s%s", c.cidr.String(), owner, name))
+	reason := ""
+	if err != nil {
+		reason = fmt.Sprintf(" - %s", err)
+	}
+	c.sendResult(fmt.Errorf("address %s is owned by other peer %s%s%s", c.cidr.String(), owner, name, reason))
 }
 
 func (c *claim) Cancel() {