Add warnings/errors to Dialogic 2.0

[coppolaemilio]

List of errors to display

Send a proper descriptive error when

• [x] trying to play a non existing timeline
• [x] specific display nodes that are somewhat needed aren't found (e.g. PortraitHolder, Music (if music is played, etc.)
• [ ] Print an error if the addons folder where Dialogic is installed is in uppercase (Addons will make the exported game not to work)
• [ ] Show an error when you are using an outdated version of Godot (like 3.4 for instance)
• [ ] Warnings for trying to multiply, add, or substract from a non int/float variable using the set variable event. Right now it fails silently and if you want to multiply a String by 3 you just get 3 as a result.

[Jowan-Spooner]

There is a ton of things that are currently not printing usefull errors. I would suggest that going forward we use the printerr() function for these, as it's clearer that something happened that was not supposed to (even if it doesn't break the game).

[coppolaemilio]

Yes! I was just creating issues as I make this example so we don't forget to add these in particular. We can modify this issue to track warnings/errors