Add warning for URL invalid media sources

[machacekj0]

Looking at filenames which Jetty rejected as illegal, for example kinetics_initial_rate_with_[b].svg. This adds a content error for the first invalid character in any bad filenames.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 40.30%. Comparing base (c5d1f1c) to head (0c424df).

Files with missing lines	Patch %	Lines
...ava/uk/ac/cam/cl/dtg/segue/etl/ContentIndexer.java	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #790      +/-   ##
==========================================
- Coverage   40.31%   40.30%   -0.01%     
==========================================
  Files         547      547              
  Lines       23773    23778       +5     
  Branches     2893     2894       +1     
==========================================
  Hits         9584     9584              
- Misses      13288    13293       +5     
  Partials      901      901              

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[sjd210]

Looks good, and works well when tested locally 👍 :)