Improve error message when running unsupported function in target-shell

[JSCU-CNI]

Removes the enormous stack trace target-shell shows when attempting to run a plugin that is not supported on the current target.

Also slightly modifies the UnsupportedPluginError message get_function throws to improve readability.

WIN-EXAMPLE:/$ msn
Unsupported function `msn` for this target with OS Windows
WIN-EXAMPLE:/$

[Schamper]

Can you add a unit test? And perhaps just use .os for the OS name?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.08%. Comparing base (bbd7fae) to head (0262d70).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1884   +/-   ##
=======================================
  Coverage   81.08%   81.08%           
=======================================
  Files         409      409           
  Lines       36706    36710    +4     
=======================================
+ Hits        29763    29767    +4     
  Misses       6943     6943           

Flag	Coverage Δ
unittests	81.08% <100.00%> (+<0.01%)	⬆️

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[codspeed-hq]

Merging this PR will not alter performance

✅ 32 untouched benchmarks

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_{Comparing JSCU-CNI:shell-unsupported-plugin (0262d70) with main (bbd7fae)}