Better exception on multiprocessing context conflicts

[ahopkins]

Since Sanic tries to set the multiprocessing context, there is a more clear error when something goes wrong. This will point the user to the docs for how to deal with the issue.

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (160ec7a) 88.217% compared to head (00f536c) 88.215%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
sanic/app.py	81.818%	1 Missing and 1 partial ⚠️
sanic/mixins/startup.py	77.777%	1 Missing and 1 partial ⚠️
sanic/mixins/listeners.py	94.736%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #2875       +/-   ##
=============================================
- Coverage   88.217%   88.215%   -0.002%     
=============================================
  Files           93        93               
  Lines         7282      7298       +16     
  Branches      1251      1254        +3     
=============================================
+ Hits          6424      6438       +14     
  Misses         600       600               
- Partials       258       260        +2     

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