improve language on selector reduce error (#794)

## Description

shorten and consolidate the error message when
the provided selectors match zero entries in the
backup entity list.

## Type of change

- [x] 🐹 Trivial/Minor

## Issue(s)

* #564

## Test Plan

- [x] 💪 Manual
- [ ] ⚡ Unit test
- [ ] 💚 E2E

src/cli/backup/exchange.go

@@ -388,7 +388,7 @@ func detailsExchangeCmd(cmd *cobra.Command, args []string) error {
 
 	ds := sel.Reduce(ctx, d)
 	if len(ds.Entries) == 0 {
-		Info(ctx, "nothing to display: no items in the backup match the provided selectors")
+		Info(ctx, selectors.ErrorNoMatchingItems)
 		return nil
 	}
 

src/internal/operations/restore.go

@@ -134,7 +134,7 @@ func (op *RestoreOperation) Run(ctx context.Context) (err error) {
 		// format the details and retrieve the items from kopia
 		fds = er.Reduce(ctx, d)
 		if len(fds.Entries) == 0 {
-			return errors.New("nothing to restore: no items in the backup match the provided selectors")
+			return selectors.ErrorNoMatchingItems
 		}
 
 	case selectors.ServiceOneDrive:

src/pkg/selectors/selectors.go

@@ -21,7 +21,10 @@ const (
 	ServiceOneDrive                // OneDrive
 )
 
-var ErrorBadSelectorCast = errors.New("wrong selector service type")
+var (
+	ErrorBadSelectorCast = errors.New("wrong selector service type")
+	ErrorNoMatchingItems = errors.New("no items match the specified selectors")
+)
 
 const (
 	scopeKeyCategory   = "category"