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/inline_completion/src/inline_completion.rs

@@ -1,7 +1,7 @@
 use std::ops::Range;
 use std::str::FromStr as _;
 
-use anyhow::{Result, anyhow};
+use anyhow::{Context as _, Result};
 use gpui::http_client::http::{HeaderMap, HeaderValue};
 use gpui::{App, Context, Entity, SharedString};
 use language::Buffer;
@@ -69,15 +69,15 @@ impl EditPredictionUsage {
     pub fn from_headers(headers: &HeaderMap<HeaderValue>) -> Result<Self> {
         let limit = headers
             .get(EDIT_PREDICTIONS_USAGE_LIMIT_HEADER_NAME)
-            .ok_or_else(|| {
-                anyhow!("missing {EDIT_PREDICTIONS_USAGE_LIMIT_HEADER_NAME:?} header")
+            .with_context(|| {
+                format!("missing {EDIT_PREDICTIONS_USAGE_LIMIT_HEADER_NAME:?} header")
             })?;
         let limit = UsageLimit::from_str(limit.to_str()?)?;
 
         let amount = headers
             .get(EDIT_PREDICTIONS_USAGE_AMOUNT_HEADER_NAME)
-            .ok_or_else(|| {
-                anyhow!("missing {EDIT_PREDICTIONS_USAGE_AMOUNT_HEADER_NAME:?} header")
+            .with_context(|| {
+                format!("missing {EDIT_PREDICTIONS_USAGE_AMOUNT_HEADER_NAME:?} header")
             })?;
         let amount = amount.to_str()?.parse::<i32>()?;