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/collab/src/db.rs

@@ -5,7 +5,7 @@ mod tables;
 pub mod tests;
 
 use crate::{Error, Result, executor::Executor};
-use anyhow::anyhow;
+use anyhow::{Context as _, anyhow};
 use collections::{BTreeMap, BTreeSet, HashMap, HashSet};
 use dashmap::DashMap;
 use futures::StreamExt;
@@ -320,11 +320,9 @@ impl Database {
 
         let mut tx = Arc::new(Some(tx));
         let result = f(TransactionHandle(tx.clone())).await;
-        let Some(tx) = Arc::get_mut(&mut tx).and_then(|tx| tx.take()) else {
-            return Err(anyhow!(
-                "couldn't complete transaction because it's still in use"
-            ))?;
-        };
+        let tx = Arc::get_mut(&mut tx)
+            .and_then(|tx| tx.take())
+            .context("couldn't complete transaction because it's still in use")?;
 
         Ok((tx, result))
     }
@@ -344,11 +342,9 @@ impl Database {
 
         let mut tx = Arc::new(Some(tx));
         let result = f(TransactionHandle(tx.clone())).await;
-        let Some(tx) = Arc::get_mut(&mut tx).and_then(|tx| tx.take()) else {
-            return Err(anyhow!(
-                "couldn't complete transaction because it's still in use"
-            ))?;
-        };
+        let tx = Arc::get_mut(&mut tx)
+            .and_then(|tx| tx.take())
+            .context("couldn't complete transaction because it's still in use")?;
 
         Ok((tx, result))
     }
@@ -853,9 +849,7 @@ fn db_status_to_proto(
                 )
             }
             _ => {
-                return Err(anyhow!(
-                    "Unexpected combination of status fields: {entry:?}"
-                ));
+                anyhow::bail!("Unexpected combination of status fields: {entry:?}");
             }
         };
     Ok(proto::StatusEntry {