Let worker return early instead of throw on unknown messages

[DeMoorJasper]

Currently the parcel workerfarm throws an error if it receives a message from an unknown worker.
In theory this is the correct behaviour, as plugins should extend the workerfarm instead of create their own once #1235 lands.
However in practise this appears to occur quite often for some reason. Haven't been able to debug it as I can't seem to reproduce.

Closes #1259

[codecov-io]

Codecov Report

Merging #1260 into master will increase coverage by 3.12%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1260      +/-   ##
==========================================
+ Coverage    84.4%   87.53%   +3.12%     
==========================================
  Files          77       77              
  Lines        3906     4108     +202     
==========================================
+ Hits         3297     3596     +299     
+ Misses        609      512      -97

Impacted Files	Coverage Δ
src/workerfarm/Worker.js	68.04% <0%> (-0.71%)	⬇️
src/utils/loadPlugins.js	64.28% <0%> (-35.72%)	⬇️
src/transforms/htmlnano.js	72.22% <0%> (-27.78%)	⬇️
src/transforms/posthtml.js	84.37% <0%> (-15.63%)	⬇️
src/assets/HTMLAsset.js	85.14% <0%> (-14.86%)	⬇️
src/workerfarm/errorUtils.js	64.7% <0%> (-5.89%)	⬇️
src/assets/CSSAsset.js	82.72% <0%> (-5.68%)	⬇️
src/assets/GLSLAsset.js	93.1% <0%> (-3.2%)	⬇️
src/assets/GraphqlAsset.js	100% <0%> (ø)	⬆️
... and 16 more

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