pbkdf2: Clarify error handling and panic conditions.

[briansmith]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 86.76471% with 9 lines in your changes missing coverage. Please review.

Project coverage is 97.00%. Comparing base (2985668) to head (1383b7d).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/error/too_much_output_requested.rs	0.00%	5 Missing ⚠️
src/pbkdf2.rs	93.33%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2222      +/-   ##
==========================================
- Coverage   97.04%   97.00%   -0.04%     
==========================================
  Files         161      162       +1     
  Lines       20415    20457      +42     
  Branches      458      458              
==========================================
+ Hits        19811    19845      +34     
- Misses        495      504       +9     
+ Partials      109      108       -1     

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