let AutoDoc set an exit code in the case of failure

[ThomasBreuer]

The idea is the same as in gap-system/gap/pull/5835, but here we deal with package documentation:
When building a manual, collect the info messages printed by MakeGAPDocDoc, use a heuristic to decide whether the result is acceptable, and let makedoc.g report failure if not; this can be used in automated tests.

For that, the function AutoDoc now ~~returns a record; up to now, it had no return value.~~sets a nonzero exit code if applicable.

This change will automatically become available in all packages that use makedoc.g containing AutoDoc calls. This should not cause problems because the exit code of processing makedoc.g did not have a meaning before the change.

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 2 lines in your changes missing coverage. Please review.

Project coverage is 79.89%. Comparing base (5f19ad0) to head (3426245).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
gap/Magic.gi	88.23%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #280      +/-   ##
==========================================
+ Coverage   79.83%   79.89%   +0.06%     
==========================================
  Files          13       13              
  Lines        2241     2258      +17     
==========================================
+ Hits         1789     1804      +15     
- Misses        452      454       +2     

Files with missing lines	Coverage Δ
gap/Magic.gd	100.00% <ø> (ø)
makedoc.g	100.00% <100.00%> (ø)
gap/Magic.gi	70.99% <88.23%> (+0.72%)	⬆️

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• Flaky Tests Detection - Detect and resolve failed and flaky tests

[ThomasBreuer]

@fingolfin
From the viewpoint of https://github.com/gap-system/ReleaseTools/blob/master/release-gap-package, it is not a good idea to change AutoDoc to return something:
The makedoc.g files of packages that call AutoDoc typically do not suppress its output, and the release-gap-package script collects and evaluates all output obtained by processing makedoc.g.
Thus the proposed change would make release-gap-package's work more complicated, instead of simplifying the task.
I will change the approach (again) ...