Change assert to log message and return
Change assert to log message and return

(cherry picked from commit 3b8808f82f88894e6cfc8549eed3b85af588b5de)

uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend