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/reqwest_client/src/reqwest_client.rs

@@ -220,7 +220,7 @@ impl http_client::HttpClient for ReqwestClient {
         req: http::Request<http_client::AsyncBody>,
     ) -> futures::future::BoxFuture<
         'static,
-        Result<http_client::Response<http_client::AsyncBody>, anyhow::Error>,
+        anyhow::Result<http_client::Response<http_client::AsyncBody>>,
     > {
         let (parts, body) = req.into_parts();