Clean tests, setup lint tools, improve errors

[rinchsan]

No description provided.

[codecov-io]

Codecov Report

Merging #1 (90a0a06) into master (a3cfb5d) will decrease coverage by 0.07%.
The diff coverage is 88.46%.

@@            Coverage Diff             @@
##           master       #1      +/-   ##
==========================================
- Coverage   90.85%   90.78%   -0.08%     
==========================================
  Files           9        9              
  Lines         164      141      -23     
==========================================
- Hits          149      128      -21     
  Misses          8        8              
+ Partials        7        5       -2     

Impacted Files	Coverage Δ
client.go	100.00% <ø> (ø)
config.go	100.00% <ø> (ø)
jwt.go	83.33% <50.00%> (-3.34%)	⬇️
query.go	78.26% <76.47%> (-9.24%)	⬇️
validate.go	82.35% <83.33%> (-5.89%)	⬇️
update.go	84.21% <85.71%> (-5.27%)	⬇️
api.go	100.00% <100.00%> (+18.75%)	⬆️
credential.go	100.00% <100.00%> (ø)
error.go	100.00% <100.00%> (ø)
... and 2 more

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