[mwtrew]

• Demo video

This PR introduces an error modal that's shown when the project fails to save due to an oversized asset. The modal tells the user that they need to remove the asset, and that saving the project will not be possible until they do so.

Implementation

Scratch's onShowAlert doesn't receive any information about why the save failed, so this approach patches Scratch's storage.store in order to be able to save error details to Redux at the time the upload fails. That allows us to determine whether the alert was caused by an oversized asset failing to upload, and show our error message accordingly.

Considerations

• The error message appears on project save, not when the asset is added. This is because we're relying on the backend to decide whether the asset is oversized. In practice, our auto-save does kick in shortly after the upload.
• We don't name the problematic asset in the error message. This would be difficult to do as the Scratch UI renames the asset to a hash before attempting to save it.
• We don't automatically remove the oversized asset from the client-side project. One reason for this is that it's possible to create assets in the editor itself (e.g. recording sounds) - if we automatically removed such assets from the client-side project on a failed project save they would be lost permanently. This solution gives users a chance to work around it, for example by downloading the project.