avoid traceback with --debuglog=[bad file]

If we can't open the debuglog, you should get an error message, not a
traceback.

Resolves: rhbz#1175480
remove --reboot, improve/reorder warning messages

--reboot was kind of a dumb idea. If you really want to ignore any
potential problems and just start the upgrade, you can do:

   fedup [args] && reboot

and you'll get the same behavior.

This lets us easily move the "Finished. Reboot to start upgrade."
message *after* the warnings, so people stop thinking that the warnings
indicate failure. (See rhbz#1162408, rhbz#1098886, rhbz#1174472, etc.)

Also add a bit about how to abort the upgrade, and make the package
problems sound less dire.