downgrade invalid callee error for event handlers to a warning

[Conduitry]

Fixes #579.

One thing is that this does downgrade the error to a warning for <:Window> components as well. Arguably we know exactly what's available on the <:Window> component and so perhaps that should still be an error. We could add an additional boolean parameter to validateEventHandler and pass in true/false in validateElement and validateWindow so we know whether to make that an error or a warning. But that all seems a bit ugly and asymmetrical.

[codecov-io]

Codecov Report

Merging #580 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #580   +/-   ##
=======================================
  Coverage   94.45%   94.45%           
=======================================
  Files          95       95           
  Lines        2779     2779           
=======================================
  Hits         2625     2625           
  Misses        154      154

Impacted Files	Coverage Δ
src/validate/html/validateEventHandler.js	93.33% <ø> (ø)	⬆️

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