[API BREAKING] Replace {:error, :invalid_repository} response with error.

[scouten]

Changes in This Pull Request

IMHO passing a PID that is not a Repository process is a developer error, and should be treated as such.

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. n/a
• 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 #264 into master will decrease coverage by 0.07%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #264      +/-   ##
==========================================
- Coverage     100%   99.92%   -0.08%     
==========================================
  Files          27       27              
  Lines        1346     1336      -10     
==========================================
- Hits         1346     1335      -11     
- Misses          0        1       +1

Impacted Files	Coverage Δ
lib/xgit/repository/plumbing.ex	99.37% <100%> (-0.63%)	⬇️
lib/xgit/repository/working_tree.ex	100% <100%> (ø)	⬆️
lib/xgit/repository/storage.ex	100% <100%> (ø)	⬆️

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