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/language_models/src/provider/cloud.rs

@@ -615,7 +615,7 @@ impl CloudLanguageModel {
                     }
                 }
 
-                return Err(anyhow!("Forbidden"));
+                anyhow::bail!("Forbidden");
             } else if status.as_u16() >= 500 && status.as_u16() < 600 {
                 // If we encounter an error in the 500 range, retry after a delay.
                 // We've seen at least these in the wild from API providers:
@@ -626,10 +626,10 @@ impl CloudLanguageModel {
                 if retries_remaining == 0 {
                     let mut body = String::new();
                     response.body_mut().read_to_string(&mut body).await?;
-                    return Err(anyhow!(
+                    anyhow::bail!(
                         "cloud language model completion failed after {} retries with status {status}: {body}",
                         Self::MAX_RETRIES
-                    ));
+                    );
                 }
 
                 Timer::after(retry_delay).await;