ErrNotExists uses PathSegment; use in more places.

ipldfree.Node is now a much better implementation of Node.

In particular, this means it will work correctly when combined with the
typed.Node wrapper implementations which expect to be able to use
ErrNotExists for logic purposes.
Signed-off-by: Eric Myhre <hash@exultant.us>

errors.go

@@ -49,7 +49,7 @@ func (e ErrWrongKind) Error() string {
 // a field can *never* exist (as differentiated from a map key which is
 // simply absent in some data).
 type ErrNotExists struct {
-	Segment string // REVIEW: might be better to use PathSegment, but depends on another refactor.
+	Segment PathSegment
 }
 
 func (e ErrNotExists) Error() string {

impl/free/freeNode.go

@@ -158,7 +158,7 @@ func (n *Node) LookupString(pth string) (ipld.Node, error) {
 	case ipld.ReprKind_Map:
 		v, exists := n._map[pth]
 		if !exists {
-			return nil, fmt.Errorf("404")
+			return nil, ipld.ErrNotExists{ipld.PathSegmentOfString(pth)}
 		}
 		return v, nil
 	case ipld.ReprKind_Invalid,
@@ -184,7 +184,7 @@ func (n *Node) Lookup(key ipld.Node) (ipld.Node, error) {
 		}
 		v, exists := n._map[ks]
 		if !exists {
-			return nil, fmt.Errorf("404")
+			return nil, ipld.ErrNotExists{ipld.PathSegmentOfString(ks)}
 		}
 		return v, nil
 	default:
@@ -196,10 +196,10 @@ func (n *Node) LookupIndex(idx int) (ipld.Node, error) {
 	switch n.kind {
 	case ipld.ReprKind_List:
 		if idx >= len(n._arr) {
-			return nil, fmt.Errorf("404")
+			return nil, ipld.ErrNotExists{ipld.PathSegmentOfInt(idx)}
 		}
 		if n._arr[idx] == nil {
-			return nil, fmt.Errorf("404")
+			return nil, ipld.ErrNotExists{ipld.PathSegmentOfInt(idx)}
 		}
 		return n._arr[idx], nil
 	case ipld.ReprKind_Invalid,