Improve theme error informations

[riquito]

I couldn't figure out why my local changes to the theme colors definition were not being picked up, so I went to the source.
Turned out my yaml file was using some improper fields.

This PR does:

• print a specific warning if in the config color.theme was set to custom and no colors.yaml (or colors.yml) was found
• print a specific warning if a color theme was found, but the parsing failed, and why it failed.

Contrary to before, if a color file definition exists and it cannot be parsed we immediately switch to the default (plus a warning). The previous logic was hiding format errors if there were other file extensions to try.

Some examples of the output (before regular lsd output):

if color.y(a)ml is found, but contains wrong data

Warning: cannot load custom theme. Theme file format invalid. date: unknown field `days-old`, expected one of `hour-old`, `day-old`, `older` at line 18 column 3.

if color.theme is set to "custom" in the config file, but no color.y(a)ml is present

Warning: cannot load custom theme. Cannot read theme file. No such file or directory (os error 2).

Sidenotes:

• warning output bleed into cargo test. This was already happening with Warning: the 'themes' directory is deprecated, use 'colors.yaml' instead., I made it slightly worse. Perhaps in general warnings should be behind #[cfg(not(test))] or similar, didn't investigate in print_error either, considered it out of scope.
• imho we should accept a specific extension for color.yml and the others. Iterating over extensions produces subtle bugs like this and there are no particular benefits (if the documentation tells you to create <filename.yml>, you create <filename.yml> - now you also get a warning)

---

TODO

• Use cargo fmt
• Add necessary tests
• Update default config/theme in README (if applicable)
• Update man page at lsd/doc/lsd.md (if applicable)

[muniu-bot]

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This pull-request has been approved by: riquito

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[riquito]

Is there anything I need to do to improve this PR?

[zwpaper]

hi @riquito, sorry for the late reply, the PR looks good and I have started the CI.

thanks for the contribution!

[codecov]

Codecov Report

❌ Patch coverage is 96.55172% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 85.87%. Comparing base (89659e4) to head (cb9930b).
⚠️ Report is 94 commits behind head on main.

Files with missing lines	Patch %	Lines
src/theme.rs	96.15%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #947      +/-   ##
==========================================
+ Coverage   85.76%   85.87%   +0.11%     
==========================================
  Files          51       51              
  Lines        5001     5026      +25     
==========================================
+ Hits         4289     4316      +27     
+ Misses        712      710       -2     

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