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_model/src/language_model.rs

@@ -8,7 +8,7 @@ mod telemetry;
 #[cfg(any(test, feature = "test-support"))]
 pub mod fake_provider;
 
-use anyhow::{Result, anyhow};
+use anyhow::{Context as _, Result};
 use client::Client;
 use futures::FutureExt;
 use futures::{StreamExt, future::BoxFuture, stream::BoxStream};
@@ -122,12 +122,16 @@ impl RequestUsage {
     pub fn from_headers(headers: &HeaderMap<HeaderValue>) -> Result<Self> {
         let limit = headers
             .get(MODEL_REQUESTS_USAGE_LIMIT_HEADER_NAME)
-            .ok_or_else(|| anyhow!("missing {MODEL_REQUESTS_USAGE_LIMIT_HEADER_NAME:?} header"))?;
+            .with_context(|| {
+                format!("missing {MODEL_REQUESTS_USAGE_LIMIT_HEADER_NAME:?} header")
+            })?;
         let limit = UsageLimit::from_str(limit.to_str()?)?;
 
         let amount = headers
             .get(MODEL_REQUESTS_USAGE_AMOUNT_HEADER_NAME)
-            .ok_or_else(|| anyhow!("missing {MODEL_REQUESTS_USAGE_AMOUNT_HEADER_NAME:?} header"))?;
+            .with_context(|| {
+                format!("missing {MODEL_REQUESTS_USAGE_AMOUNT_HEADER_NAME:?} header")
+            })?;
         let amount = amount.to_str()?.parse::<i32>()?;
 
         Ok(Self { limit, amount })