capi: Use camelcase for FizzyErrorCode members

[axic]

Renamed as discussed.

[codecov]

Codecov Report

Merging #783 (330d078) into master (5b99b52) will not change coverage.
The diff coverage is 92.85%.

@@           Coverage Diff           @@
##           master     #783   +/-   ##
=======================================
  Coverage   99.24%   99.24%           
=======================================
  Files          79       79           
  Lines       11962    11962           
=======================================
  Hits        11872    11872           
  Misses         90       90           

Flag	Coverage Δ
rust	99.87% <ø> (ø)
spectests	90.54% <ø> (ø)
unittests	99.20% <92.85%> (ø)

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Impacted Files	Coverage Δ
lib/fizzy/capi.cpp	97.53% <66.66%> (ø)
test/unittests/capi_test.cpp	99.88% <100.00%> (ø)