Hooks for intercepting uploads, outputting feedback pertaining to uploaded file

Hello,

I’d like to add a hook/filter to provide my own code to examine an upload attachment, and either delete it while outputting an error message to the browser, or else permit it and provide some separate output to the browser.

I see I can use the wp_handle_upload filter to handle all of the backend stuff (and I’m assuming having it return false will block the upload?), but what is available to provide the output to the browser?

In case the context here is relevant, I’m using wp_handle_upload to setup a queue for encoding video into h.264 and WebM depending on the input format and uploading it to Amazon S3. All of this is working, but I want to say either “please wait x minutes for your video to be transcoded”, or “sorry, you cannot upload any more videos” once a limit has been reached.

Thanks in advance for your help!