fix: Avoid .unwrap() when running the discover command

[Wilfred]

Previously, the following configuration in settings.json:

"rust-analyzer.workspace.discoverConfig": {
    "command": [
        "oops",
        "develop-json",
        "{arg}"
    ],
    "progressLabel": "rust-analyzer",
    "filesToWatch": [
        "BUCK",
        "TARGETS"
    ]
},

Would previously cause a crash in rust-analyzer:

thread 'LspServer' panicked at crates/rust-analyzer/src/main_loop.rs:776:84:
called `Result::unwrap()` on an `Err` value: Os { code: 2, kind: NotFound, message: "No such file or directory" }

Instead, log an error and proceed. This gives the user a useful hint as to what's broken, and ensures we have a basic rust-analyzer running.

[ChayimFriedman2]

I feel like crashing is acceptable here? If we can't run the command, we can't discover the project, and r-a doesn't have a lot it can do.

The choice of the temp dir also feels arbitrary and may cause weird misbehaviors.

[flodiebold]

We don't crash when e.g. cargo metadata fails, right?

[Wilfred]

Logging a specific error and r-a continuing in detached file mode does feel more friendly than just "rust-analyzer crashed five times, it will not restart" which is the current behaviour.

I noticed this when I'd configured the discover command with a local build, forgot about it, eventually cleaned up my build directory, and got very confused when my r-a setup broke.

[Wilfred]

Regarding the fallback directory, I considered / but temp_dir() has the advantage that it works on Windows too. I do think the spawn() failing is much more likely, but I figured I'd remove both .unwrap() calls for cleanliness.

[davidbarsky]

My weak opinion here is that we should probably crash five times if the provided argument doesn't exist/buck2 did some some garbage collection on buck-out. Maybe panic with a more descriptive error message?

Don't get me wrong: I don't think the current state is good, but we could probably make this suck less with an .expect("Unable to run project discovery command") and get us 80% of the way there...