Move warning for missing Rust extension into function body

[puddly]

I've noticed the warning logs a bit more often than necessary. We currently do not rely on the Rust module for anything other than listing serial ports so this can be made into a loud runtime warning.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.70%. Comparing base (ff630dd) to head (e7f199a).

Files with missing lines	Patch %	Lines
serialx/serialx_rust.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev      #77      +/-   ##
==========================================
- Coverage   90.73%   90.70%   -0.03%     
==========================================
  Files          22       22              
  Lines        3367     3367              
==========================================
- Hits         3055     3054       -1     
- Misses        312      313       +1     

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