mintupdate: crash when /var/lib/apt/lists missing or empty

[ripleymj]

tl;dr - mintupdate errors when /var/lib/apt/lists is missing or empty. Tray icon shows as this:

and running mintupdate from a terminal shows this as the GUI is frozen:

-- Exception occurred in the refresh thread:
Traceback (most recent call last):
  File "/usr/lib/linuxmint/mintUpdate/mintUpdate.py", line 830, in run
    self.application.builder.get_object("label_error_details").set_text(error_msg)
TypeError: Argument 1 does not allow None as a value

after a manual apt-get update, mintupdate appears to function normally.

Longer story - we build a Mint appliance VM and try to optimize for space savings, including removing the package lists. This has worked for us since at least Mint Ulyana but now fails with the Uma beta.