restore and verify report an unknown snapshot ID as not found (closes #426)

A snapshot ID that names no snapshot in the local index or on the
destination store made restore and verify fail on a missing object at
a hashed path, and verify printed a header for it first. Resolving a
snapshot ID now uses the look-up `snapshot remove` already does, so
all three commands give the same error, and verify prints its header
only after the snapshot is found.

Model: opus-5-5

internal/vaultik/snapshot.go

@@ -1477,7 +1477,7 @@ func (v *Vaultik) RemoveSnapshot(
 		return result, err
 	}
 
-	inLocalIndex, err := v.checkSnapshotExists(snapshotID, opts.LocalOnly)
+	inLocalIndex, err := v.findSnapshot(snapshotID, opts.LocalOnly)
 	if err != nil {
 		return result, err
 	}
@@ -1540,20 +1540,15 @@ func (v *Vaultik) RemoveSnapshot(
 	return result, nil
 }
 
-// checkSnapshotExists returns errSnapshotNotFound when the local index
-// has no row for snapshotID and, unless localOnly is set, the
-// destination store has no metadata for it either. inLocalIndex reports
-// whether the local index has that row. The destination store is
-// consulted only when the local index has no row, so a snapshot in the
-// local index can still be removed from it while the destination store
-// is unreachable, and only after checkDestinationMarker passes, so an
-// unplugged volume's empty mount point is not taken for a store without
-// the snapshot.
-//
-//nolint:nonamedreturns // the result name says what the bool means
-func (v *Vaultik) checkSnapshotExists(
-	snapshotID string, localOnly bool,
-) (inLocalIndex bool, err error) {
+// findSnapshot reports whether the local index has a row for
+// snapshotID, and returns errSnapshotNotFound when there is no row and,
+// unless localOnly is set, no metadata on the destination store either.
+// The destination store is consulted only when the local index has no
+// row, so a snapshot in the local index can still be removed from it
+// while the destination store is unreachable, and only after
+// checkDestinationMarker passes, so an unplugged volume's empty mount
+// point is not taken for a store without the snapshot.
+func (v *Vaultik) findSnapshot(snapshotID string, localOnly bool) (bool, error) {
 	if v.Repositories != nil {
 		snap, err := v.Repositories.Snapshots.GetByID(v.ctx, snapshotID)
 		if err != nil {
@@ -1570,7 +1565,7 @@ func (v *Vaultik) checkSnapshotExists(
 			errSnapshotNotFound, snapshotID)
 	}
 
-	err = v.checkDestinationMarker()
+	err := v.checkDestinationMarker()
 	if err != nil {
 		return false, err
 	}

internal/vaultik/snapshot_identifier.go

@@ -30,7 +30,7 @@ var (
 //   - A human snapshot ID (hostname_name_timestamp), which a host holding
 //     the local index has. It is hashed to its remote key. An ID that
 //     names no snapshot in the local index or on the destination store
-//     is an error (see checkSnapshotExists).
+//     is an error (see findSnapshot).
 //   - A remote key, or the leading part of one, which is all a host with
 //     no local index can know — it is exactly what `snapshot list` prints
 //     for a remote-only snapshot (see formatRemoteOnlyID). It is resolved
@@ -42,7 +42,7 @@ var (
 // RFC3339 timestamp).
 func (v *Vaultik) resolveSnapshotRemoteKey(identifier string) (string, error) {
 	if !isRemoteKeyOrPrefix(identifier) {
-		_, err := v.checkSnapshotExists(identifier, false)
+		_, err := v.findSnapshot(identifier, false)
 		if err != nil {
 			return "", err
 		}