Fix return value from HTTP 400s.

[pattivacek]

No description provided.

[codecov-io]

Codecov Report

Merging #929 into master will decrease coverage by 0.41%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #929      +/-   ##
==========================================
- Coverage   81.74%   81.33%   -0.42%     
==========================================
  Files         184      185       +1     
  Lines       11755    12443     +688     
==========================================
+ Hits         9609    10120     +511     
- Misses       2146     2323     +177

Impacted Files	Coverage Δ
src/sota_tools/ostree_http_repo.cc	1.58% <0%> (ø)	⬆️
src/libaktualizr/primary/sotauptaneclient.cc	72.86% <0%> (-27.14%)	⬇️
src/libaktualizr/http/httpclient.cc	77.12% <0%> (-7.85%)	⬇️
src/sota_tools/ostree_dir_repo.cc	73.52% <0%> (-5.89%)	⬇️
src/libaktualizr/crypto/keymanager.cc	89.65% <0%> (-2.07%)	⬇️
src/libaktualizr/uptane/directorrepository.h	100% <0%> (ø)	⬆️
src/libaktualizr/uptane/uptanerepository.h	100% <0%> (ø)	⬆️
src/libaktualizr/primary/sotauptaneclient.h	100% <0%> (ø)	⬆️
src/libaktualizr/uptane/imagesrepository.h	100% <0%> (ø)	⬆️
src/libaktualizr/bootloader/bootloader.h	100% <0%> (ø)
... and 5 more

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