Improve no such node error messages

model.go

@@ -12,7 +12,6 @@ acquire locks, but document what locks they require.
 */
 
 import (
-	"errors"
 	"fmt"
 	"os"
 	"path"
@@ -40,10 +39,6 @@ type Model struct {
 	lastIdxBcastRequest time.Time
 }
 
-var (
-	errNoSuchNode = errors.New("no such node")
-)
-
 const (
 	FlagDeleted = 1 << 12
 
@@ -231,7 +226,7 @@ func (m *Model) RequestGlobal(nodeID, name string, offset uint64, size uint32, h
 	nc, ok := m.nodes[nodeID]
 	m.RUnlock()
 	if !ok {
-		return nil, errNoSuchNode
+		return nil, fmt.Errorf("RequestGlobal: no such node: %s", nodeID)
 	}
 
 	if opts.Debug.TraceNet {

model_puller.go

@@ -34,7 +34,7 @@ func (m *Model) pullFile(name string) error {
 	m.RUnlock()
 
 	if len(nodeIDs) == 0 {
-		return errNoSuchNode
+		return fmt.Errorf("%s: no connected nodes with file available", name)
 	}
 
 	filename := path.Join(m.dir, name)