Add LightningCLI(save_config_overwrite=False|True)

[carmocca]

What does this PR do?

Add overwrite to SaveConfigCallback + plumbing for it
Improve error message
Add test

cc: @mauvilsa

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[codecov]

Codecov Report

Merging #8059 (4b6097f) into master (2303f9c) will decrease coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #8059   +/-   ##
======================================
- Coverage      92%     91%   -0%     
======================================
  Files         210     210           
  Lines       13579   13581    +2     
======================================
- Hits        12452   12416   -36     
- Misses       1127    1165   +38