Loading a project with invalid extension hangs at the "loading" animation #2179
Description
Expected Behavior
If you load a project with an invalid extension (i.e. containing a block with an opcode for an extension that is not officially supported by Scratch), you should end up in a reasonable error state, such as the "oops" page.
Actual Behavior
Currently, if you load an sb3 containing an invalid extension, the editor or viewer gets stuck forever at the blue "loading" animation. There's a console error related to worker extensions.