Improve tools

[albertziegenhagel]

Some minor improvements & clean-up for the utility tools.

This is mainly about naming the tools "tools" instead of "apps" and improving the detection and error reporting of invalid/unsupported ETL files.

[codecov]

Codecov Report

Attention: Patch coverage is 34.78261% with 15 lines in your changes are missing coverage. Please review.

Project coverage is 83.01%. Comparing base (6d03ff8) to head (b1cc22f).

Files	Patch %	Lines
snail/etl/etl_file.cpp	34.78%	7 Missing and 8 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #60      +/-   ##
==========================================
- Coverage   83.31%   83.01%   -0.31%     
==========================================
  Files         102      102              
  Lines        4310     4322      +12     
  Branches      829      837       +8     
==========================================
- Hits         3591     3588       -3     
- Misses        339      346       +7     
- Partials      380      388       +8     

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