Show a friendly error message when starting lazygit from a non-existent cwd (#3192)

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**

* [x] Cheatsheets are up-to-date (run `go generate ./...`)
* [x] Code has been formatted (see
[here](https://github.com/jesseduffield/lazygit/blob/master/CONTRIBUTING.md#code-formatting))
* [x] Tests have been added/updated (see
[here](https://github.com/jesseduffield/lazygit/blob/master/pkg/integration/README.md)
for the integration test guide)
* [x] Text is internationalised (see
[here](https://github.com/jesseduffield/lazygit/blob/master/CONTRIBUTING.md#internationalisation))
* [x] Docs (specifically `docs/Config.md`) have been updated if
necessary
* [x] You've read through your own file changes for silly mistakes etc

<!--
Be sure to name your PR with an imperative e.g. 'Add worktrees view'
see https://github.com/jesseduffield/lazygit/releases/tag/v0.40.0 for
examples
-->

pkg/app/errors.go

@@ -27,6 +27,10 @@ func knownError(tr *i18n.TranslationSet, err error) (string, bool) {
 			originalError: "fatal: not a git repository",
 			newError:      tr.NotARepository,
 		},
+		{
+			originalError: "getwd: no such file or directory",
+			newError:      tr.WorkingDirectoryDoesNotExist,
+		},
 	}
 
 	if mapping, ok := lo.Find(mappings, func(mapping errorMapping) bool {

pkg/i18n/english.go

@@ -335,6 +335,7 @@ type TranslationSet struct {
 	StashIncludeUntrackedChanges        string
 	StashOptions                        string
 	NotARepository                      string
+	WorkingDirectoryDoesNotExist        string
 	Jump                                string
 	ScrollLeftRight                     string
 	ScrollLeft                          string
@@ -1153,6 +1154,7 @@ func EnglishTranslationSet() TranslationSet {
 		StashIncludeUntrackedChanges:        "Stash all changes including untracked files",
 		StashOptions:                        "Stash options",
 		NotARepository:                      "Error: must be run inside a git repository",
+		WorkingDirectoryDoesNotExist:        "Error: the current working directory does not exist",
 		Jump:                                "Jump to panel",
 		ScrollLeftRight:                     "Scroll left/right",
 		ScrollLeft:                          "Scroll left",