fix(engine-ffi): return error to caller on connection or parse json failures

[erka]

resolves #253

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 94.24460% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 85.90%. Comparing base (f562a7d) to head (2a55f37).

Files	Patch %	Lines
flipt-engine-ffi/src/evaluator/mod.rs	94.82%	6 Missing ⚠️
flipt-engine-ffi/src/parser/http.rs	0.00%	1 Missing ⚠️
flipt-evaluation/src/parser/mod.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #254      +/-   ##
==========================================
+ Coverage   82.29%   85.90%   +3.60%     
==========================================
  Files          11       11              
  Lines        3152     3277     +125     
==========================================
+ Hits         2594     2815     +221     
+ Misses        558      462      -96     

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[markphelps]

Closes: #253

[markphelps]

cheers thanks @erka ! one minor nit about naming of the empty state method for Snapshot

[markphelps]

awesome! ty!!