Replace assertions with error messages

[abhro]

Draft until all the asserts are replaced, and also when all the error messages are deemed suitable by a reviewer.

[codecov]

Codecov Report

❌ Patch coverage is 46.66667% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.23%. Comparing base (63fb577) to head (3aa18d6).

Files with missing lines	Patch %	Lines
src/ASDF.jl	46.66%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #20      +/-   ##
==========================================
- Coverage   95.28%   93.23%   -2.06%     
==========================================
  Files           1        1              
  Lines         318      325       +7     
==========================================
  Hits          303      303              
- Misses         15       22       +7     

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