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/eval/src/eval.rs

@@ -11,7 +11,6 @@ use instance::{ExampleInstance, JudgeOutput, RunOutput, run_git};
 pub(crate) use tool_metrics::*;
 
 use ::fs::RealFs;
-use anyhow::anyhow;
 use clap::Parser;
 use client::{Client, ProxySettings, UserStore};
 use collections::{HashMap, HashSet};
@@ -255,13 +254,10 @@ fn main() {
 
                         let actual_origin =
                             run_git(&repo_path, &["remote", "get-url", "origin"]).await?;
-                        if actual_origin != repo_url {
-                            return Err(anyhow!(
-                                "remote origin {} does not match expected origin {}",
-                                actual_origin,
-                                repo_url,
-                            ));
-                        }
+                        anyhow::ensure!(
+                            actual_origin == repo_url,
+                            "remote origin {actual_origin} does not match expected origin {repo_url}"
+                        );
                     }
                 }
             }
@@ -467,7 +463,7 @@ pub fn find_model(
 
     match matching_models.as_slice() {
         [model] => Ok(model.clone()),
-        [] => Err(anyhow!(
+        [] => anyhow::bail!(
             "No language model with ID {}/{} was available. Available models: {}",
             provider_id,
             model_id,
@@ -476,15 +472,15 @@ pub fn find_model(
                 .map(|model| format!("{}/{}", model.provider_id().0, model.id().0))
                 .collect::<Vec<_>>()
                 .join(", ")
-        )),
-        _ => Err(anyhow!(
+        ),
+        _ => anyhow::bail!(
             "Multiple language models with ID {} available - use `--provider` to choose one of: {:?}",
             model_id,
             matching_models
                 .iter()
                 .map(|model| model.provider_id().0)
                 .collect::<Vec<_>>()
-        )),
+        ),
     }
 }