Description

This PR aims to fix #3984. It improves media upload error handling, by showing a generic message when the upload fails, and by validating the max file size before starting the upload so we are not wasting resources try to send an invalid file to the server.

How Has This Been Tested?

Add an image block. Try to upload an image bigger than max file size upload limit. the value can be changed by adding this lines "php_value upload_max_filesize 3M / php_value post_max_size 3M" in .htaccess.
See that the correct error notice appears.

Screenshots (jpeg or gifs if applicable):

Types of changes

Added an onError handler to media handler. Used the handler to validate max file size and generic upload problems.
Passed max upload size from in client assets to allow max file size validation.
Added the ability for blocks to display error notices.