Inaccurate error message in ui.image when file does not exist

[CrystalWindSnake]

When using ui.image, if the parameter is a non-existent file path, the following error message will appear in the backend. It is difficult to determine the exact source of the error. This PR fixes this .

from nicegui import ui
from pathlib import Path

missing_path = Path('missing.png')
ui.image(missing_path)

error(not contain useful information to locate the source of the error):

TypeError: Type is not JSON serializable: WindowsPath

[falkoschindler]

Thanks for the pull request, @CrystalWindSnake! The previous error message was indeed not very helpful.
I moved the check from ImageElement into the SourceElement mixin to make it effective for many more UI elements like ui.audio, ui.video etc.:

ui.audio(Path('foo.mp3'))