Fix and disallow mdbook errors

[mdboom]

When mdbook reports errors, it doesn't actually return a UNIX error code. So there were include directives inserted into the docs that were "broken" but still passed CI.

I filed this issue with mdbook: rust-lang/mdBook#1094

In the meantime, we can use grep to detect errors and return an error code from CI.

[codecov-io]

Codecov Report

Merging #466 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #466      +/-   ##
============================================
+ Coverage     76.68%   76.69%   +0.01%     
  Complexity      330      330              
============================================
  Files            96       96              
  Lines          5554     5557       +3     
  Branches        630      631       +1     
============================================
+ Hits           4259     4262       +3     
  Misses          831      831              
  Partials        464      464

Impacted Files	Coverage Δ	Complexity Δ
...a/telemetry/glean/net/HttpURLConnectionUploader.kt	91.11% <0%> (+0.63%)	14% <0%> (ø)	⬇️

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