Detect broken apt and ask users to fix it first

[fmarier]

A user with an already-broken apt won't be able to use our installer script. Maybe we should check early that apt-get update works without errors before we make any changes.

If it doesn't work, then we can print out a useful error along the lines of "There is a problem with apt on your system. It needs to be fixed before you can install Brave."

See https://community.brave.com/t/installation-on-ubuntu-failed/600506.