monitor: improve error messages

Print more understandable messages on error:

- When ps fails because the monitor doesn't attach to any session, print "no
  attaching session" instead of "unknown ref".
- Avoid disconnect silently fails when the monitor doesn't attach to any
  session. Print "no attaching session" error instead.
- Fix error message of "attach"'s arguments. ("server name must be passed" ->
  "ID of session or process must be passed")

Signed-off-by: Kohei Tokunaga <ktokunaga.mail@gmail.com>

monitor/commands/ps.go

@@ -7,6 +7,7 @@ import (
 	"text/tabwriter"
 
 	"github.com/docker/buildx/monitor/types"
+	"github.com/pkg/errors"
 )
 
 type PsCmd struct {
@@ -24,6 +25,9 @@ func (cm *PsCmd) Info() types.CommandInfo {
 
 func (cm *PsCmd) Exec(ctx context.Context, args []string) error {
 	ref := cm.m.AttachedSessionID()
+	if ref == "" {
+		return errors.Errorf("no attaching session")
+	}
 	plist, err := cm.m.ListProcesses(ctx, ref)
 	if err != nil {
 		return err