Better user feedback for CSV/Excel downloads

[jan-niestadt]

Exporting a full dataset to CSV/Excel can take a long time, and can sometimes fail. There is a spinner animation while the download is being prepared, but no other user feedback.

• when a user starts the download, there should probably be a message under the buttons saying e.g. "Download is being prepared; this may take a few minutes..."
• if the download fails (maybe the server timed out, or something else failed), it should display an error message. (note that the spinner does stop on error, so it should be easy to hook into there to display the error?)

[jan-niestadt]

It might even be better to freeze the application while the export happens, otherwise the user might think it's okay to continue using the application and accidentally cancel the download.