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/prompt_store/src/prompt_store.rs

@@ -1,6 +1,6 @@
 mod prompts;
 
-use anyhow::{Result, anyhow};
+use anyhow::{Context as _, Result, anyhow};
 use chrono::{DateTime, Utc};
 use collections::HashMap;
 use futures::FutureExt as _;
@@ -266,10 +266,7 @@ impl PromptStore {
         let bodies = self.bodies;
         cx.background_spawn(async move {
             let txn = env.read_txn()?;
-            let mut prompt = bodies
-                .get(&txn, &id)?
-                .ok_or_else(|| anyhow!("prompt not found"))?
-                .into();
+            let mut prompt = bodies.get(&txn, &id)?.context("prompt not found")?.into();
             LineEnding::normalize(&mut prompt);
             Ok(prompt)
         })