[libfuzzer] Improve error message for file not found (#207081)

We're seeing many instances of this issue on ClusterFuzz. This PR adds
more information to help debug the cause of the problem.

See https://crbug.com/529865766 for more context.

GitOrigin-RevId: f74953b929fefb8edef80f6e99d596b72b540c66
1 file changed