fix: handle broken link for the missing image file

[Chadys]

We get an exception when the missing image file has previously been symlinked and for whatever reason the link got broken (it happened to me after upgrading the python version of my project).
When media_file_path is a link but does not exists as a file, this is because the symlink got broken. In that case we can destroy the symlink before trying to link it again (which fails if we don't unlink it first).

[codecov]

Codecov Report

Merging #4287 (430c303) into master (204b9a7) will increase coverage by 0.07%.
Report is 11 commits behind head on master.
The diff coverage is 50.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4287      +/-   ##
==========================================
+ Coverage   88.28%   88.35%   +0.07%     
==========================================
  Files         293      293              
  Lines       16046    16096      +50     
==========================================
+ Hits        14166    14222      +56     
+ Misses       1880     1874       -6     

Files	Coverage Δ
src/oscar/apps/catalogue/abstract_models.py	93.41% <50.00%> (-0.13%)	⬇️

... and 9 files with indirect coverage changes