Show a friendly error message when starting lazygit from a non-existent cwd

[simonwhitaker]

Closes #3187

• PR Description

#3187 observes that lazygit crashes with a stack trace if it's run from a non-existent current working directory. The steps to reproduce are:

mkdir test
cd test
rm -r ../test
lazygit

(Note: I can repro this on Ubuntu, but not on macOS Sonoma, where lazygit starts regardless of whether the current working directory exists or not.)

Here's how the repro steps look on Ubuntu with this PR applied:

simon@ubuntu:/Users/simon/src/3p/lazygit$ go build .
simon@ubuntu:/Users/simon/src/3p/lazygit$ mkdir deleteme
simon@ubuntu:/Users/simon/src/3p/lazygit$ cd deleteme/
simon@ubuntu:/Users/simon/src/3p/lazygit/deleteme$ rm -r ../deleteme
simon@ubuntu:/Users/simon/src/3p/lazygit/deleteme$ ../lazygit
2024/01/02 18:40:15 Error: the current working directory does not exist

• Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• Docs (specifically docs/Config.md) have been updated if necessary
• You've read through your own file changes for silly mistakes etc

[jesseduffield]

LGTM, good work

(Also just realised we should have squashed those commits before merging. Sorry @stefanhaller haha)