check: Suggest usage of restic repair packs for corrupted blobs

For now, the guide is only shown if the blob content does not match its
hash. The main intended usage is to handle data corruption errors when
using maximum compression in restic 0.16.0

internal/checker/checker.go

@@ -90,6 +90,16 @@ func (err *ErrOldIndexFormat) Error() string {
 	return fmt.Sprintf("index %v has old format", err.ID)
 }
 
+// ErrPackData is returned if errors are discovered while verifying a packfile
+type ErrPackData struct {
+	PackID restic.ID
+	errs   []error
+}
+
+func (e *ErrPackData) Error() string {
+	return fmt.Sprintf("pack %v contains %v errors: %v", e.PackID, len(e.errs), e.errs)
+}
+
 func (c *Checker) LoadSnapshots(ctx context.Context) error {
 	var err error
 	c.snapshots, err = backend.MemorizeList(ctx, c.repo.Backend(), restic.SnapshotFile)
@@ -635,7 +645,7 @@ func checkPack(ctx context.Context, r restic.Repository, id restic.ID, blobs []r
 	}
 
 	if len(errs) > 0 {
-		return errors.Errorf("pack %v contains %v errors: %v", id, len(errs), errs)
+		return &ErrPackData{PackID: id, errs: errs}
 	}
 
 	return nil