More careful error checking around key generation

[lbonn]

As discussed in #1487

[codecov-io]

Codecov Report

Merging #1499 into master will increase coverage by 0.11%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #1499      +/-   ##
==========================================
+ Coverage   80.67%   80.78%   +0.11%     
==========================================
  Files         184      184              
  Lines       11148    11150       +2     
==========================================
+ Hits         8994     9008      +14     
+ Misses       2154     2142      -12

Impacted Files	Coverage Δ
src/libaktualizr/crypto/crypto.cc	86.11% <100%> (+0.05%)	⬆️
src/uptane_generator/repo.cc	95.63% <50%> (-0.42%)	⬇️
src/libaktualizr/storage/sql_utils.h	84.5% <0%> (-1.41%)	⬇️
src/libaktualizr/storage/sqlstorage.cc	76.56% <0%> (+0.22%)	⬆️
src/libaktualizr/package_manager/ostreemanager.cc	80.14% <0%> (+1.12%)	⬆️
src/libaktualizr/storage/sqlstorage_base.cc	79.05% <0%> (+4.72%)	⬆️
src/libaktualizr/storage/sqlstorage_base.h	100% <0%> (+40%)	⬆️

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