print download failures cleanly
    
    Currently they are unhandled exceptions which means abrt is triggered,
    even though a download failure is not actually a bug.
    
    (cherry picked from commit 6c6c3075bd479951ba88af914293422f2cc58735)
    
    Bug: 1423678
    Change-Id: Ieb39c6849cd68b4c5657be0564443c7df603c96c