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/extension_host/src/extension_host.rs

@@ -717,7 +717,7 @@ impl ExtensionStore {
             let mut response = http_client
                 .get(url.as_ref(), Default::default(), true)
                 .await
-                .map_err(|err| anyhow!("error downloading extension: {}", err))?;
+                .context("downloading extension")?;
 
             fs.remove_dir(
                 &extension_dir,
@@ -1415,7 +1415,7 @@ impl ExtensionStore {
         let is_dev = fs
             .metadata(&extension_dir)
             .await?
-            .ok_or_else(|| anyhow!("directory does not exist"))?
+            .context("directory does not exist")?
             .is_symlink;
 
         if let Ok(mut language_paths) = fs.read_dir(&extension_dir.join("languages")).await {