Default error message, when message is empty

[Joshua Powers]

Description:
During one of my install attempts an error occurred and an empty error message window popped up. This is less than helpful, as I then hit enter, which rebooted the system. If an empty message gets detected then there should be some default message that says how to get additional information.

Actual Result:
Empty error, see: https://imgur.com/a/n4vF3
subiquity-debug.log: https://paste.ubuntu.com/26399406/
curtin install.log: https://paste.ubuntu.com/26399408/

Expected Result:
A message detailing how to find additional help for example:

"Oops we have no error message to display, but an error occured. Could you please drop to the shell and run the following commands and include the links to both logs in a bug on http://launchpad.net/subiquity/+filebug

pastebinit /var/log/installer/subiquity-debug.log
pastebinit /var/log/curtin/install.log"

Steps to reproduce:
1. wget latest live-iso
2. This occured when I was trying an install with lots of disks and network devices

[Michael Hudson-Doyle (mwhudson)]

I think I've just found a problem where no message would be displayed if curtin crashed quickly enough. Having said that, a default message in the case of no error would definitely be an improvement.

[Michael Hudson-Doyle (mwhudson)]

I haven't actually fixed this bug as suggested but I have fixed all the ways I can think that curtin can fail and we don't get an error and a new bug can be opened if that starts happening again.