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/tests/random_project_collaboration_tests.rs

@@ -1,6 +1,6 @@
 use super::{RandomizedTest, TestClient, TestError, TestServer, UserTestPlan};
 use crate::{db::UserId, tests::run_randomized_test};
-use anyhow::{Result, anyhow};
+use anyhow::{Context as _, Result};
 use async_trait::async_trait;
 use call::ActiveCall;
 use collections::{BTreeMap, HashMap};
@@ -782,8 +782,7 @@ impl RandomizedTest for ProjectCollaborationTest {
                 let save =
                     project.update(cx, |project, cx| project.save_buffer(buffer.clone(), cx));
                 let save = cx.spawn(|cx| async move {
-                    save.await
-                        .map_err(|err| anyhow!("save request failed: {:?}", err))?;
+                    save.await.context("save request failed")?;
                     assert!(
                         buffer
                             .read_with(&cx, |buffer, _| { buffer.saved_version().to_owned() })