Add Optional Error Context

[hodovani]

Add optional length of error context.

Resolves #13
Resolves #3

[codecov]

Codecov Report

Merging #46 into master will increase coverage by 0.6%.
The diff coverage is 90.24%.

@@            Coverage Diff            @@
##           master      #46     +/-   ##
=========================================
+ Coverage    55.4%   56.01%   +0.6%     
=========================================
  Files          29       29             
  Lines        1397     1414     +17     
=========================================
+ Hits          774      792     +18     
+ Misses        623      622      -1

Impacted Files	Coverage Δ
Sources/XMLCoder/Encoder/XMLEncoder.swift	47.57% <ø> (ø)	⬆️
Sources/XMLCoder/Decoder/XMLDecoder.swift	38.64% <100%> (-1.17%)	⬇️
Sources/XMLCoder/Box/NullBox.swift	100% <100%> (ø)	⬆️
Sources/XMLCoder/Auxiliaries/XMLStackParser.swift	91.78% <88.88%> (+5.24%)	⬆️

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[regexident]

FYI this crashes if errorPosition - offset < 0 or errorPosition + offset > string.length.

And yes, it's missing some test that might have caught these scenarios. 😉

[MaxDesiatov]

Thanks for reporting @regexident! @hodovani could you please have a look and add corresponding test cases and fixes? Thanks