Handle a nil transfer better

Sometimes `pgbackup_client.get_transfer(transfer_id)` ends up nil, so this
commit will handle that more gracefully. Don't try to update the display with a
nil transfer, instead, just let things loop again

Fixes https://github.com/heroku/heroku/issues/896
Merge pull request #967 from catsby/nil_transfer_fix

Handle a nil transfer better