Made image loading failure in preload() not to stop whole process

[r21nomi]

Resolves #5032

Issue

• setup(), draw() was not called if an image loading in preload() failed
  • It's inconvenient for the case like the image was on server and network connection issue

Changes

• Made image loading failure in preload() not to stop whole process
  • setup(), draw() will be called if there was an failure callback in p5.loadImage()
  • setup(), draw() will not be called if there was no failure callback in p5.loadImage()

I know @limzykenneth says this might be breaking changes.
So please check this changes are right way or not.

PR Checklist

• npm run lint passes
• Inline documentation is included / updated
• Unit tests are included / updated

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[limzykenneth]

I think this is fine to fix now that I had a look at it. There is no easy semantic way to continue the sketch after a handled error halted the sketch and a error halted sketch will probably not be counted as a semantic sketch so changing its behaviour won't be a breaking change.

#5292 Can be fix in the same manner as well (along with other load functions with the same behaviour)