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/google.rs

@@ -279,7 +279,7 @@ impl GoogleLanguageModel {
         };
 
         async move {
-            let api_key = api_key.ok_or_else(|| anyhow!("Missing Google API key"))?;
+            let api_key = api_key.context("Missing Google API key")?;
             let request = google_ai::stream_generate_content(
                 http_client.as_ref(),
                 &api_url,
@@ -351,7 +351,7 @@ impl LanguageModel for GoogleLanguageModel {
         let api_url = settings.api_url.clone();
 
         async move {
-            let api_key = api_key.ok_or_else(|| anyhow!("Missing Google API key"))?;
+            let api_key = api_key.context("Missing Google API key")?;
             let response = google_ai::count_tokens(
                 http_client.as_ref(),
                 &api_url,