RAPPS - Display error messages on download failures

I see in many cases, if a download in Rapps fails, it will fail silently (the dialog with the progress bar simply closes). Unless a user is actively watching the progress bar, they won't know if the download didn't complete successfully, or if the completed download didn't execute as it should.

This patch adds specific messages (MessageBox) for the 'unable to write to disk' and 'download interrupted' failures.