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/context_store.rs

@@ -2,7 +2,7 @@ use std::ops::Range;
 use std::path::{Path, PathBuf};
 use std::sync::Arc;
 
-use anyhow::{Result, anyhow};
+use anyhow::{Context as _, Result, anyhow};
 use assistant_context_editor::AssistantContext;
 use collections::{HashSet, IndexSet};
 use futures::{self, FutureExt};
@@ -142,17 +142,12 @@ impl ContextStore {
         remove_if_exists: bool,
         cx: &mut Context<Self>,
     ) -> Result<Option<AgentContextHandle>> {
-        let Some(project) = self.project.upgrade() else {
-            return Err(anyhow!("failed to read project"));
-        };
-
-        let Some(entry_id) = project
+        let project = self.project.upgrade().context("failed to read project")?;
+        let entry_id = project
             .read(cx)
             .entry_for_path(project_path, cx)
             .map(|entry| entry.id)
-        else {
-            return Err(anyhow!("no entry found for directory context"));
-        };
+            .context("no entry found for directory context")?;
 
         let context_id = self.next_context_id.post_inc();
         let context = AgentContextHandle::Directory(DirectoryContextHandle {