OTA-3717: Yield an error if root.json is malformed

[mike-sul]

It makes aktualizr behave properly if root.json is malformed (invalid json, or missing mandatory field).

But it doesn't fix the case when director or imagerepo is hacked and a hacker set the version filed to 1 in root.json.

Signed-off-by: Mykhaylo Sul myk.sul@gmail.com

[codecov-io]

Codecov Report

Merging #1417 into master will decrease coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1417      +/-   ##
==========================================
- Coverage   80.41%   80.38%   -0.04%     
==========================================
  Files         181      181              
  Lines       10642    10648       +6     
==========================================
+ Hits         8558     8559       +1     
- Misses       2084     2089       +5

Impacted Files	Coverage Δ
src/libaktualizr/uptane/directorrepository.cc	95.89% <100%> (+0.17%)	⬆️
src/libaktualizr/uptane/imagesrepository.cc	89.09% <100%> (+0.2%)	⬆️
src/libaktualizr/storage/sqlstorage_base.cc	79.05% <0%> (-1.36%)	⬇️
src/libaktualizr/storage/sqlstorage.cc	76.89% <0%> (-1.04%)	⬇️
src/libaktualizr/primary/sotauptaneclient.cc	89.25% <0%> (+0.14%)	⬆️
src/aktualizr_info/main.cc	92.11% <0%> (+0.49%)	⬆️
src/libaktualizr/package_manager/ostreemanager.cc	79.08% <0%> (+1.52%)	⬆️

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