Partial unpack

[meparle]

1. Return a partially unpacked message if there is an error during unpack()
2. Add Field to UnpackError to indicate in which field the error occurred without needing to retrieve from within the error message

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (8f4cb18) 74.85% compared to head (aadd842) 74.86%.

❗ Current head aadd842 differs from pull request most recent head 03ff030. Consider uploading reports for the commit 03ff030 to get more accurate results

Files	Patch %	Lines
message.go	80.00%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #300      +/-   ##
==========================================
+ Coverage   74.85%   74.86%   +0.01%     
==========================================
  Files          44       44              
  Lines        2494     2495       +1     
==========================================
+ Hits         1867     1868       +1     
  Misses        411      411              
  Partials      216      216              

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