improve error message if no apps found in registry

[scardozos]

Fixes #2579

[codecov]

Codecov Report

Base: 88.018% // Head: 88.093% // Increases project coverage by +0.075% 🎉

Coverage data is based on head (5044ea3) compared to base (97f33f4).
Patch has no changes to coverable lines.

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #2585       +/-   ##
=============================================
+ Coverage   88.018%   88.093%   +0.075%     
=============================================
  Files           71        71               
  Lines         5333      5333               
  Branches       893       893               
=============================================
+ Hits          4694      4698        +4     
+ Misses         461       459        -2     
+ Partials       178       176        -2     

Impacted Files	Coverage Δ
sanic/server/websockets/impl.py	38.061% <0.000%> (+0.945%)	⬆️

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