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/anthropic/src/anthropic.rs

@@ -90,7 +90,7 @@ impl Model {
         } else if id.starts_with("claude-3-haiku") {
             Ok(Self::Claude3Haiku)
         } else {
-            Err(anyhow!("invalid model id"))
+            anyhow::bail!("invalid model id {id}");
         }
     }
 
@@ -385,10 +385,10 @@ impl RateLimitInfo {
     }
 }
 
-fn get_header<'a>(key: &str, headers: &'a HeaderMap) -> Result<&'a str, anyhow::Error> {
+fn get_header<'a>(key: &str, headers: &'a HeaderMap) -> anyhow::Result<&'a str> {
     Ok(headers
         .get(key)
-        .ok_or_else(|| anyhow!("missing header `{key}`"))?
+        .with_context(|| format!("missing header `{key}`"))?
         .to_str()?)
 }