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/src/extension_manifest.rs

@@ -1,4 +1,4 @@
-use anyhow::{Context as _, Result, anyhow, bail};
+use anyhow::{Context as _, Result, bail};
 use collections::{BTreeMap, HashMap};
 use fs::Fs;
 use language::LanguageName;
@@ -213,7 +213,7 @@ impl ExtensionManifest {
         let extension_name = extension_dir
             .file_name()
             .and_then(OsStr::to_str)
-            .ok_or_else(|| anyhow!("invalid extension name"))?;
+            .context("invalid extension name")?;
 
         let mut extension_manifest_path = extension_dir.join("extension.json");
         if fs.is_file(&extension_manifest_path).await {