Refactor error handling

[robo9k]

Partially implements #112

CookieDatabaseError might not be needed if the foo<P: AsRef<Path>>(&self, filenames: &[P]) were e.g. validated with a TryInto for DatabaseFilePaths.

[codecov]

Codecov Report

Attention: 96 lines in your changes are missing coverage. Please review.

Comparison is base (f954fec) 41.69% compared to head (bec91ae) 37.23%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #113      +/-   ##
==========================================
- Coverage   41.69%   37.23%   -4.47%     
==========================================
  Files           2        2              
  Lines         506      564      +58     
  Branches      506      564      +58     
==========================================
- Hits          211      210       -1     
- Misses        289      350      +61     
+ Partials        6        4       -2     

Files	Coverage Δ
src/ffi.rs	47.87% <18.75%> (-5.07%)	⬇️
src/lib.rs	31.91% <17.39%> (-4.10%)	⬇️

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[robo9k]

I'm don't have a good idea how to test the high- and medium-level code with the real libmagic below. Especially the assertions are hopefully only theoretical.

Postponing this and thus test coverage for whenever.