beginShape() and endShape() needs more helpful error messages

[trych]

I am writing this down as a reminder for later:

One common error, that happens all the time

// beginShape() is not outside the for-loop as it should be
for(var i = 0; i < 20; i++) {
  beginShape();
  vertex(random(width), random(height);
}
endShape();

gives the error Type error: null is not an object. Could do with a more helpful error.

Another error I just had in class was Error: entire path. Still need to investigate the script in question to find out what went wrong there.