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/thread.rs

@@ -1630,7 +1630,7 @@ impl Thread {
                                         CompletionRequestStatus::Failed {
                                             code, message, request_id
                                         } => {
-                                            return Err(anyhow!("completion request failed. request_id: {request_id}, code: {code}, message: {message}"));
+                                            anyhow::bail!("completion request failed. request_id: {request_id}, code: {code}, message: {message}");
                                         }
                                         CompletionRequestStatus::UsageUpdated {
                                             amount, limit