error if message ids mismatch

[nicksnyder]

helps #125

cc @bep

[codecov]

Codecov Report

Merging #132 into master will decrease coverage by 0.03%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #132      +/-   ##
==========================================
- Coverage   67.79%   67.76%   -0.04%     
==========================================
  Files          28       28              
  Lines        2282     2286       +4     
==========================================
+ Hits         1547     1549       +2     
- Misses        657      659       +2     
  Partials       78       78

Impacted Files	Coverage Δ
v2/i18n/localizer.go	81.81% <50%> (-1.52%)	⬇️

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