flows: return correct status code on error

[BeryJu]

currently we return status 200 for everything; errors are structured the same as in any other API endpoint

closes #18294

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 42.79%. Comparing base (64965e3) to head (242f2c5).
✅ All tests successful. No failed tests found.

❗ There is a different number of reports uploaded between BASE (64965e3) and HEAD (242f2c5). Click for more details.

HEAD has 24 uploads less than BASE

Flag	BASE (64965e3)	HEAD (242f2c5)
e2e	8	4
unit	10	0
unit-migrate	10	0

Additional details and impacted files

@@             Coverage Diff             @@
##             main   #18293       +/-   ##
===========================================
- Coverage   92.96%   42.79%   -50.18%     
===========================================
  Files         869      863        -6     
  Lines       48143    47650      -493     
===========================================
- Hits        44756    20390    -24366     
- Misses       3387    27260    +23873     

Flag	Coverage Δ
e2e	41.85% <100.00%> (-3.31%)	⬇️
integration	23.11% <0.00%> (-0.06%)	⬇️
unit	?
unit-migrate	?

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