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/agent/src/thread_store.rs

@@ -419,7 +419,7 @@ impl ThreadStore {
             let thread = database
                 .try_find_thread(id.clone())
                 .await?
-                .ok_or_else(|| anyhow!("no thread found with ID: {id:?}"))?;
+                .with_context(|| format!("no thread found with ID: {id:?}"))?;
 
             let thread = this.update_in(cx, |this, window, cx| {
                 cx.new(|cx| {
@@ -699,20 +699,14 @@ impl SerializedThread {
                 SerializedThread::VERSION => Ok(serde_json::from_value::<SerializedThread>(
                     saved_thread_json,
                 )?),
-                _ => Err(anyhow!(
-                    "unrecognized serialized thread version: {}",
-                    version
-                )),
+                _ => anyhow::bail!("unrecognized serialized thread version: {version:?}"),
             },
             None => {
                 let saved_thread =
                     serde_json::from_value::<LegacySerializedThread>(saved_thread_json)?;
                 Ok(saved_thread.upgrade())
             }
-            version => Err(anyhow!(
-                "unrecognized serialized thread version: {:?}",
-                version
-            )),
+            version => anyhow::bail!("unrecognized serialized thread version: {version:?}"),
         }
     }
 }