Fix a bug where data files are deleted if GetAsync fails when updating

[pmachapman]

Fixes #779

I also took the opportunity to update DataFileService with some modern dotnet features (await using), and to clean up some of the formatting.

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This change is 

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 66.10%. Comparing base (a3eee4b) to head (52ef516).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #787      +/-   ##
==========================================
- Coverage   66.13%   66.10%   -0.03%     
==========================================
  Files         370      370              
  Lines       20019    20012       -7     
  Branches     2617     2619       +2     
==========================================
- Hits        13239    13229      -10     
- Misses       5850     5852       +2     
- Partials      930      931       +1     

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[ddaspit]

@ddaspit reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @Enkidu93)

[Enkidu93]

@Enkidu93 reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @pmachapman)