internal/cmd/docgen: use "missingkey=error"

[mmcloughlin]

No description provided.

[codecov-commenter]

Codecov Report

Merging #287 (2efee0d) into master (62c7110) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #287   +/-   ##
=======================================
  Coverage   76.05%   76.05%           
=======================================
  Files          65       65           
  Lines       20595    20595           
=======================================
  Hits        15663    15663           
  Misses       4854     4854           
  Partials       78       78           

Flag	Coverage Δ
integration	11.84% <ø> (ø)
unittests	73.16% <ø> (ø)

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