Better error on OS watcher limit

[samuelcolvin]

ref #207

[codecov]

Codecov Report

Merging #208 (c76976e) into main (94ef903) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #208   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         5    -1     
  Lines          397       391    -6     
  Branches        82        82           
=========================================
- Hits           397       391    -6     

Impacted Files	Coverage Δ
watchfiles/__init__.py

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