Add 'pre-flight' checks for Onivim 2

[bryphe]

This is tracking a follow-up for discussion on #189 - we want to have infrastructure to make a few checks when the app starts to establish some invariants (like a home directory is available).

For now, if these pre-flight checks fail - we'll quit the app with an actionable error message. Later, once we've addressed #81 , we might be able to have a nicer experience.