capi: better OOM handling

[gumb0]

No description provided.

[codecov]

Codecov Report

Merging #788 (81d3813) into master (81efc84) will decrease coverage by 0.03%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master     #788      +/-   ##
==========================================
- Coverage   99.25%   99.22%   -0.04%     
==========================================
  Files          79       79              
  Lines       12042    12060      +18     
==========================================
+ Hits        11952    11966      +14     
- Misses         90       94       +4     

Flag	Coverage Δ
rust	99.87% <ø> (ø)
spectests	90.54% <ø> (ø)
unittests	99.17% <77.77%> (-0.04%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
test/unittests/oom_test.cpp	88.09% <75.00%> (-8.06%)	⬇️
lib/fizzy/capi.cpp	97.56% <100.00%> (+0.01%)	⬆️

[axic]

Looks okay to me, but not fully convinced we want to expose this as a separate error.