Add "error" as reserved username

[lafriks]

It is already used in code in multiple places (ex mustOwner function that returns owner name "error" when owner can not be loaded), so it should be added as reserved not to have problems at some point

[codecov-io]

Codecov Report

Merging #3882 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3882      +/-   ##
==========================================
- Coverage   20.18%   20.17%   -0.02%     
==========================================
  Files         145      145              
  Lines       29151    29151              
==========================================
- Hits         5883     5880       -3     
- Misses      22374    22376       +2     
- Partials      894      895       +1

Impacted Files	Coverage Δ
models/user.go	23.49% <ø> (ø)	⬆️
modules/process/manager.go	69.56% <0%> (-4.35%)	⬇️

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[lunny]

LGTM