Enhance assets processing reliability and logs

[benoit74]

Fix #84
Fix #89

Changes:

• revisit all logs to display as much information as possible
• enrich assets metadata (in memory) to be able to log where they are used
• catch all exceptions arising while processing an asset, not only RequestsException (if there is a bad bug, the asset threshold is intended to catch it anyway)

[codecov]

Codecov Report

Attention: Patch coverage is 18.18182% with 27 lines in your changes missing coverage. Please review.

Project coverage is 49.20%. Comparing base (1399a4a) to head (de9e0ee).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
scraper/src/mindtouch2zim/processor.py	6.25%	15 Missing ⚠️
scraper/src/mindtouch2zim/asset.py	25.00%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #96      +/-   ##
==========================================
- Coverage   49.42%   49.20%   -0.23%     
==========================================
  Files          17       17              
  Lines        1050     1063      +13     
  Branches      147      149       +2     
==========================================
+ Hits          519      523       +4     
- Misses        520      529       +9     
  Partials       11       11              

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[rgaudin]

No actual change request but I'd like you to explain it to me (live)