Add error message to Nelder-Mead optimizer when user cost function returns NaN value

[henrygerardmoore]

Hello, I just had a bug in my cost function where it returned NaN and got the unreachable point error in the Nelder-Mead optimizer. Of course this is my fault 😁 but I thought it'd be nice to have an error message directly stating this since it's the only case I can think of that should hit that unreachable point message so that a user that makes my same mistake in the future can see exactly why they got it without debugging.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.68%. Comparing base (b300285) to head (9619de3).

Files with missing lines	Patch %	Lines
crates/argmin/src/solver/neldermead/mod.rs	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #635      +/-   ##
==========================================
- Coverage   91.72%   91.68%   -0.04%     
==========================================
  Files         177      177              
  Lines       20840    20849       +9     
==========================================
  Hits        19116    19116              
- Misses       1724     1733       +9     

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