[24.0] Don't log exception if cancelled slurm job doesn't have stderr file

[mvdbeek]

This seems entirely expected and not worth reporting at an ERROR level. Fixes https://sentry.galaxyproject.org/share/issue/c7332e0cf1554c33b2cf56283acfcd1d/:

FileNotFoundError
[Errno 2] No such file or directory: '/corral4/main/jobs/057/797/57797989/galaxy_57797989.e'

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