Use anyhow more idiomatically (#31052)

https://github.com/zed-industries/zed/issues/30972 brought up another
case where our context is not enough to track the actual source of the
issue: we get a general top-level error without inner error.

The reason for this was `.ok_or_else(|| anyhow!("failed to read HEAD
SHA"))?; ` on the top level.

The PR finally reworks the way we use anyhow to reduce such issues (or
at least make it simpler to bubble them up later in a fix).
On top of that, uses a few more anyhow methods for better readability.

* `.ok_or_else(|| anyhow!("..."))`, `map_err` and other similar error
conversion/option reporting cases are replaced with `context` and
`with_context` calls
* in addition to that, various `anyhow!("failed to do ...")` are
stripped with `.context("Doing ...")` messages instead to remove the
parasitic `failed to` text
* `anyhow::ensure!` is used instead of `if ... { return Err(...); }`
calls
* `anyhow::bail!` is used instead of `return Err(anyhow!(...));`

Release Notes:

- N/A

tooling/xtask/src/tasks/clippy.rs

@@ -1,6 +1,6 @@
 use std::process::Command;
 
-use anyhow::{Context, Result, bail};
+use anyhow::{Context as _, Result, bail};
 use clap::Parser;
 
 #[derive(Parser)]

tooling/xtask/src/tasks/licenses.rs

@@ -1,6 +1,6 @@
 use std::path::{Path, PathBuf};
 
-use anyhow::{Result, anyhow};
+use anyhow::{Context as _, Result};
 use clap::Parser;
 
 use crate::workspace::load_workspace;
@@ -17,7 +17,7 @@ pub fn run_licenses(_args: LicensesArgs) -> Result<()> {
         let crate_dir = package
             .manifest_path
             .parent()
-            .ok_or_else(|| anyhow!("no crate directory for {}", package.name))?;
+            .with_context(|| format!("no crate directory for {}", package.name))?;
 
         if let Some(license_file) = first_license_file(crate_dir, LICENSE_FILES) {
             if !license_file.is_symlink() {

tooling/xtask/src/tasks/package_conformity.rs

@@ -2,7 +2,7 @@ use std::collections::BTreeMap;
 use std::fs;
 use std::path::Path;
 
-use anyhow::{Context as _, Result, anyhow};
+use anyhow::{Context as _, Result};
 use cargo_toml::{Dependency, Manifest};
 use clap::Parser;
 
@@ -78,5 +78,5 @@ fn read_cargo_toml(path: impl AsRef<Path>) -> Result<Manifest> {
     let path = path.as_ref();
     let cargo_toml_bytes = fs::read(path)?;
     Manifest::from_slice(&cargo_toml_bytes)
-        .with_context(|| anyhow!("failed to read Cargo.toml at {path:?}"))
+        .with_context(|| format!("reading Cargo.toml at {path:?}"))
 }