Use error wrapping

Closes #26.

main.go

@@ -74,7 +74,7 @@ func backupLoop(uploader *uploader.Uploader, monitor *monitor.Monitor, timeout t
 			}
 			cancel()
 		case err := <-monitor.Errors:
-			return fmt.Errorf("monitor error: %v", err)
+			return fmt.Errorf("monitor error: %w", err)
 		case <-done:
 			return nil
 		}

monitor/monitor.go

@@ -85,11 +85,11 @@ func (m *Monitor) Close() error {
 func New(dir string) (*Monitor, error) {
 	watcher, err := fsnotify.NewWatcher()
 	if err != nil {
-		return nil, fmt.Errorf("failed to create watcher: %v", err)
+		return nil, fmt.Errorf("failed to create watcher: %w", err)
 	}
 	if err = watcher.Add(dir); err != nil {
 		watcher.Close()
-		return nil, fmt.Errorf("failed to watch %v: %v", dir, err)
+		return nil, fmt.Errorf("failed to watch %v: %w", dir, err)
 	}
 	return &Monitor{
 		watcher: watcher,

uploader/uploader.go

@@ -88,7 +88,7 @@ func (u *Uploader) Upload(ctx context.Context, path string) (*s3manager.UploadOu
 	f, err := os.Open(path)
 	if err != nil {
 		uploadFailures.Inc()
-		return nil, fmt.Errorf("failed to open %v for uploading: %v", path, err)
+		return nil, fmt.Errorf("failed to open %v for uploading: %w", path, err)
 	}
 	defer f.Close()
 
@@ -103,7 +103,7 @@ func (u *Uploader) Upload(ctx context.Context, path string) (*s3manager.UploadOu
 	})
 	if err != nil {
 		uploadFailures.Inc()
-		return nil, fmt.Errorf("failed to upload %v: %v", base, err)
+		return nil, fmt.Errorf("failed to upload %v: %w", base, err)
 	}
 	elapsed := time.Since(start)
 	uploadDuration.Set(elapsed.Seconds())