[std] Unconventional `// error` comments

[eisenwave]

Searching for the regex // .*error, yields 7 results currently, such as:

draft/source/templates.tex

Line 6742 in 2a7f36a

h<false, int>(0); // error, instantiates \tcode{f<int>} even though \tcode{B} evaluates to \tcode{false} and

The convention is // error: (colon instead of comma), of which there are 584 occurrences.

All instances of // .*error, should be harmonized.