Localize remaining strings

[hanzei]

This PR localizes the last few remaining strings. It does that by adding a poll.ErrorMessage struct (0/5 on the naming). This message is meant for a user to consume and should not be wrapped.

I would love to hear feedback on the design.

Fixes #196

[codecov]

Codecov Report

Merging #198 into master will increase coverage by 0.1%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #198     +/-   ##
=========================================
+ Coverage   96.22%   96.33%   +0.1%     
=========================================
  Files          12       12             
  Lines         822      846     +24     
=========================================
+ Hits          791      815     +24     
  Misses         20       20             
  Partials       11       11

Impacted Files	Coverage Δ
server/plugin/command.go	100% <100%> (ø)	⬆️
server/plugin/i18n.go	90.9% <100%> (+1.16%)	⬆️
server/plugin/api.go	93.67% <100%> (+0.02%)	⬆️
server/poll/poll.go	96.93% <100%> (+0.68%)	⬆️

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[kaakaa]

Current architecture is fine for me, and naming ErrorMessage is the best for now.
I left one more comment, so please take a look it.