Exit with code 2 when an inventory file or url cannot be read and logging enhancements

[tristanlatr]

When an inventory file fails to read, log the error and continue without the information instead of printing the whole stack trace.

Improve the error reporting of intersphinx related failures. The -v flag will log exceptions' tracebacks.

Fixes #751

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.37%. Comparing base (c1e68f8) to head (d3d7125).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #942      +/-   ##
==========================================
+ Coverage   93.33%   93.37%   +0.03%     
==========================================
  Files          48       48              
  Lines        8912     8931      +19     
  Branches     1636     1639       +3     
==========================================
+ Hits         8318     8339      +21     
+ Misses        337      336       -1     
+ Partials      257      256       -1     

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[tristanlatr]

This PR is missing a change log entry

[github-actions]

Diff from pydoctor_primer, showing the effect of this PR on open source code:

twisted (https://github.com/twisted/twisted): typechecking got 1.05x slower (271.4s -> 285.9s)
(Performance measurements are based on a single noisy sample)