Use constants in place of error variables

[flimzy]

Fixes #805

[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (50c091b) 78.98% compared to head (bd6ce10) 78.96%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #814      +/-   ##
==========================================
- Coverage   78.98%   78.96%   -0.03%     
==========================================
  Files         166      166              
  Lines       13341    13333       -8     
==========================================
- Hits        10538    10528      -10     
- Misses       2364     2366       +2     
  Partials      439      439              

Files	Coverage Δ
cluster.go	100.00% <100.00%> (ø)
config.go	100.00% <100.00%> (ø)
errors.go	100.00% <ø> (+14.28%)	⬆️
find.go	100.00% <100.00%> (ø)
replication.go	100.00% <100.00%> (ø)
db.go	93.43% <33.33%> (ø)
internal/errors.go	21.73% <0.00%> (-1.34%)	⬇️

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