Fix lint errors and improve test

[rinchsan]

• Fixed golangci-lint errors
• Upgraded golangci-lint version
• Added test for invalid credential files

[codecov]

Codecov Report

Merging #35 (c1330b2) into main (849f8c0) will decrease coverage by 3.66%.
The diff coverage is 52.63%.

@@            Coverage Diff             @@
##             main      #35      +/-   ##
==========================================
- Coverage   95.77%   92.10%   -3.67%     
==========================================
  Files           9        9              
  Lines         213      228      +15     
==========================================
+ Hits          204      210       +6     
- Misses          6       12       +6     
- Partials        3        6       +3     

Impacted Files	Coverage Δ
query.go	79.06% <33.33%> (-12.60%)	⬇️
jwt.go	87.50% <40.00%> (-12.50%)	⬇️
credential.go	100.00% <100.00%> (ø)

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