Fixed rollback

[forman]

Fixed rollback for situations where writing to Zarr fails shortly after the Zarr directory has been created.

Closes #69

Checklist (strike out non-applicable):

• Changes documented in CHANGES.md
• Related issue exists and is referred to in the PR description and CHANGES.md
• Added docstrings and API docs for any new/modified user-facing classes and functions
• Changes/features documented in docs/*
• Unit-tests adapted/added for changes/features
• Test coverage remains or increases (target 100%)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.85%. Comparing base (b75706a) to head (9b5bfa7).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #70   +/-   ##
=======================================
  Coverage   99.85%   99.85%           
=======================================
  Files          26       26           
  Lines        1371     1372    +1     
=======================================
+ Hits         1369     1370    +1     
  Misses          2        2           

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[thomasstorm]

Looks fine, and additionally clearer than before.

[forman]

Thanks @thomasstorm!