[API BREAKING] ConfigFile APIs are very unlikely to return POSIX errors.

[scouten]

Changes in This Pull Request

Rather than writing code to handle them, we'll crash if it happens.

Checklist

• This PR represents a single feature, fix, or change.
• All applicable changes have been documented.
• There is test coverage for all changes.
• All cases where a literal value is returned use the cover macro to force code coverage.
• Any code ported from jgit maintains all existing copyright and license notices. n/a
• If new files are ported from jgit, the path to the corresponding file(s) is included in the header comment. n/a
• Any TO DO items (or similar) have been entered as GitHub issues and the link to that issue has been included in a comment. n/a

[codecov]

Codecov Report

Merging #312 into master will not change coverage by %.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #312   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           30        30           
  Lines         1642      1642           
=========================================
  Hits          1642      1642           

Impacted Files	Coverage Δ
lib/xgit/config_file.ex	100.00% <100.00%> (ø)

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