Exit non-zero when snapshot create --skip-errors skips anything (closes #310)

A backup under --skip-errors printed its skip errors and failed count,
then "Finished successfully.", and exited 0, so a cron wrapper could
not tell it from a complete backup. A directory the walk could not list
was not counted at all.

Each entry the walk skips is now counted as examined and failed. When
anything failed under --skip-errors, the run keeps its snapshots, runs
the post-backup prune as before, and then returns an error that the
command reports as "Backup completed, but N file(s) failed to back up".

Judgement call: without --skip-errors a file removed during the run is
still counted as failed and the exit status stays 0.

Model: opus-5-5

internal/snapshot/scanner.go

@@ -141,10 +141,11 @@ type ScannerConfig struct {
 // ScanResult contains the results of a scan operation. Files and bytes
 // are counted per file: BytesScanned is the size of the new and changed
 // files, BytesSkipped that of the unchanged ones. FilesFailed counts the
-// new and changed files that phase 2 could not store; FilesScanned
-// includes them and BytesScanned does not. ChunksCreated counts the
-// chunks that were not already stored, and NewChunkSize is their total
-// size.
+// new and changed files that phase 2 could not store and, under
+// --skip-errors, each entry the walk could not read, such as a directory
+// it could not list; FilesScanned includes them and BytesScanned does
+// not. ChunksCreated counts the chunks that were not already stored, and
+// NewChunkSize is their total size.
 type ScanResult struct {
 	FilesScanned   int
 	FilesSkipped   int
@@ -883,7 +884,7 @@ func (s *Scanner) scanPhase(
 
 	walkFn := func(filePath string, info os.FileInfo, err error) error {
 		if err != nil {
-			return s.handleWalkError(filePath, err)
+			return s.handleWalkError(filePath, err, result)
 		}
 
 		// Check context cancellation
@@ -959,7 +960,7 @@ func (s *Scanner) recordSpecialEntry(
 	if info.Mode()&os.ModeSymlink != 0 {
 		file, err := s.buildSymlinkEntry(filePath, info)
 		if err != nil {
-			return true, s.handleWalkError(filePath, err)
+			return true, s.handleWalkError(filePath, err, result)
 		}
 
 		existingFiles[filePath] = struct{}{}
@@ -983,14 +984,20 @@ func (s *Scanner) recordSpecialEntry(
 }
 
 // handleWalkError deals with a filesystem error surfaced by the walk:
-// skipped (with a UI notice) under --skip-errors, fatal otherwise.
-func (s *Scanner) handleWalkError(filePath string, err error) error {
+// under --skip-errors the entry is skipped with a UI notice and counted
+// as examined and failed, otherwise the error is fatal.
+func (s *Scanner) handleWalkError(
+	filePath string, err error, result *ScanResult,
+) error {
 	if s.skipErrors {
 		log.Error("Failed to access file (skipping due to --skip-errors)",
 			"path", filePath, "error", err)
 		s.ui.Errorf("Failed to access %s: %v. Skipping (--skip-errors).",
 			s.ui.Path(filePath), err)
 
+		result.FilesScanned++
+		result.FilesFailed++
+
 		return nil // Continue scanning
 	}