use custom err msg instead of ctx.Err()

Signed-off-by: Dr. Jan-Philip Gehrcke <[email protected]>

Author: jgehrcke

pkg/flock/flock.go

@@ -107,7 +107,7 @@ func (l *Flock) Acquire(ctx context.Context, opts ...AcquireOption) (func(), err
 		select {
 		case <-ctx.Done():
 			f.Close()
-			return nil, ctx.Err()
+			return nil, fmt.Errorf("context done before lock acquired (%s)", l.path)
 		case <-ticker.C:
 			// Retry flock().
 			continue