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/google_ai/src/google_ai.rs

@@ -39,8 +39,7 @@ pub async fn stream_generate_content(
                             match serde_json::from_str(line) {
                                 Ok(response) => Some(Ok(response)),
                                 Err(error) => Some(Err(anyhow!(format!(
-                                    "Error parsing JSON: {:?}\n{:?}",
-                                    error, line
+                                    "Error parsing JSON: {error:?}\n{line:?}"
                                 )))),
                             }
                         } else {
@@ -85,15 +84,13 @@ pub async fn count_tokens(
     let mut response = client.send(http_request).await?;
     let mut text = String::new();
     response.body_mut().read_to_string(&mut text).await?;
-    if response.status().is_success() {
-        Ok(serde_json::from_str::<CountTokensResponse>(&text)?)
-    } else {
-        Err(anyhow!(
-            "error during countTokens, status code: {:?}, body: {}",
-            response.status(),
-            text
-        ))
-    }
+    anyhow::ensure!(
+        response.status().is_success(),
+        "error during countTokens, status code: {:?}, body: {}",
+        response.status(),
+        text
+    );
+    Ok(serde_json::from_str::<CountTokensResponse>(&text)?)
 }
 
 pub fn validate_generate_content_request(request: &GenerateContentRequest) -> Result<()> {