Update error messages html comment placeholders with mustache tags

[thelovekesh]

Fixes #709

HTML comment placeholders get stripped by HTML minifiers. So update those HTML comment placeholders with Moustache tags.

[codecov-commenter]

Codecov Report

Merging #720 (da9321a) into develop (ad09837) will not change coverage.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             develop     #720   +/-   ##
==========================================
  Coverage      19.96%   19.96%           
  Complexity       326      326           
==========================================
  Files             56       56           
  Lines           2094     2094           
==========================================
  Hits             418      418           
  Misses          1676     1676           

Flag	Coverage Δ
php	19.96% <0.00%> (ø)
unit	19.96% <0.00%> (ø)

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Impacted Files	Coverage Δ
wp-admin/error.php	0.00% <0.00%> (ø)
wp-includes/template.php	11.11% <0.00%> (ø)

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