Don't raise an exception if `fuser` fails

[jwodder]

Closes #315.

[codecov[bot]]

Codecov Report

Base: 75.79% // Head: 75.79% // No change to project coverage :thumbsup:

Coverage data is based on head (6bf6ed9) compared to base (b2fa781). Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## draft #316 +/- ## ======================================= Coverage 75.79% 75.79% ======================================= Files 14 14 Lines 2384 2384 Branches 378 378 ======================================= Hits 1807 1807 Misses 427 427 Partials 150 150 ``` | [Impacted Files](https://codecov.io/gh/dandi/dandisets/pull/316?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dandi) | Coverage Δ | | |---|---|---| | [tools/backups2datalad/adataset.py](https://codecov.io/gh/dandi/dandisets/pull/316/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dandi#diff-dG9vbHMvYmFja3VwczJkYXRhbGFkL2FkYXRhc2V0LnB5) | `77.46% <0.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dandi). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dandi)

:umbrella: View full report at Codecov.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.