Error improvement when call Assets.loadEverything before System.start

[RblSb]

Currently this mistake is complicated to debug, because:

• Loading callback if fired without problems
• Rendering without resources is fine
• g2.drawImage fails with:

  Error: Cannot cast {
  g2canvas : null, 
  myWidth : 240, 
  myHeight : 160, 
  myFormat : 0, 
  renderTarget : false, 
  image : [object HTMLImageElement], 
  video : null
  } to {
  __name__ : true, 
  init : <function>, 
  formatByteSize : <function>
  }

  And endless Error: End before you begin spam.

We can throw this problem at step 1 if do not ignore null in SystemImpl.gl here 10 years ago: https://github.com/Kode/Kha/blob/2462f98275a24c24b65200a7e7fd9744339a4a11/Backends/HTML5/kha/CanvasImage.hx#L144-L146

Or maybe even earlier someway.