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

crates/indexed_docs/src/providers/rustdoc.rs

@@ -12,7 +12,7 @@ use std::path::PathBuf;
 use std::sync::{Arc, LazyLock};
 use std::time::{Duration, Instant};
 
-use anyhow::{Context, Result, bail};
+use anyhow::{Context as _, Result, bail};
 use async_trait::async_trait;
 use collections::{HashSet, VecDeque};
 use fs::Fs;

crates/indexed_docs/src/store.rs

@@ -2,7 +2,7 @@ use std::path::PathBuf;
 use std::sync::Arc;
 use std::sync::atomic::AtomicBool;
 
-use anyhow::{Result, anyhow};
+use anyhow::{Context as _, Result, anyhow};
 use async_trait::async_trait;
 use collections::HashMap;
 use derive_more::{Deref, Display};
@@ -66,7 +66,7 @@ impl IndexedDocsStore {
         let registry = IndexedDocsRegistry::global(cx);
         registry
             .get_provider_store(provider.clone())
-            .ok_or_else(|| anyhow!("no indexed docs store found for {provider}"))
+            .with_context(|| format!("no indexed docs store found for {provider}"))
     }
 
     pub fn new(
@@ -285,7 +285,7 @@ impl IndexedDocsDatabase {
             let txn = env.read_txn()?;
             entries
                 .get(&txn, &key)?
-                .ok_or_else(|| anyhow!("no docs found for {key}"))
+                .with_context(|| format!("no docs found for {key}"))
         })
     }