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/remote_server/src/headless_project.rs

@@ -1,5 +1,5 @@
 use ::proto::{FromProto, ToProto};
-use anyhow::{Result, anyhow};
+use anyhow::{Context as _, Result, anyhow};
 
 use extension::ExtensionHostProxy;
 use extension_host::headless_host::HeadlessExtensionStore;
@@ -368,7 +368,7 @@ impl HeadlessProject {
                 let mut parent = path
                     .parent()
                     .ok_or(e)
-                    .map_err(|_| anyhow!("{:?} does not exist", path))?;
+                    .with_context(|| format!("{path:?} does not exist"))?;
                 if parent == Path::new("") {
                     parent = util::paths::home_dir();
                 }
@@ -558,11 +558,7 @@ impl HeadlessProject {
         mut cx: AsyncApp,
     ) -> Result<proto::FindSearchCandidatesResponse> {
         let message = envelope.payload;
-        let query = SearchQuery::from_proto(
-            message
-                .query
-                .ok_or_else(|| anyhow!("missing query field"))?,
-        )?;
+        let query = SearchQuery::from_proto(message.query.context("missing query field")?)?;
         let results = this.update(&mut cx, |this, cx| {
             this.buffer_store.update(cx, |buffer_store, cx| {
                 buffer_store.find_search_candidates(&query, message.limit as _, this.fs.clone(), cx)