fix: announce when a config file is invalid and exit with a non-zero code

[G-Rath]

When looking into some other config stuff I realised the current loading logic just assumes that an error is because a config doesn't exist and silently falls back to the default one, when really it could be that there is a config but it's invalid

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.94%. Comparing base (19e0abe) to head (2236e21).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1242      +/-   ##
==========================================
+ Coverage   67.92%   67.94%   +0.01%     
==========================================
  Files         174      174              
  Lines       16835    16838       +3     
==========================================
+ Hits        11435    11440       +5     
+ Misses       4766     4765       -1     
+ Partials      634      633       -1     

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