Return first field encoding error encountered, if any, when encoding a struct.

[crunk1]

Addresses #472

[codecov]

Codecov Report

Merging #473 into master will decrease coverage by 0.38%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #473      +/-   ##
==========================================
- Coverage   86.64%   86.26%   -0.39%     
==========================================
  Files          41       41              
  Lines        5100     5102       +2     
==========================================
- Hits         4419     4401      -18     
- Misses        540      560      +20     
  Partials      141      141              

Impacted Files	Coverage Δ
reflect_struct_encoder.go	84.29% <100.00%> (+0.26%)	⬆️
reflect_struct_decoder.go	82.68% <0.00%> (-2.67%)	⬇️

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