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/explorer.rs

@@ -1,4 +1,4 @@
-use anyhow::{Context, Result, anyhow};
+use anyhow::{Context as _, Result};
 use clap::Parser;
 use serde_json::{Value, json};
 use std::fs;
@@ -57,7 +57,7 @@ fn inject_thread_data(template: String, threads_data: Value) -> Result<String> {
     let injection_marker = "let threadsData = window.threadsData || { threads: [dummyThread] };";
     template
         .find(injection_marker)
-        .ok_or_else(|| anyhow!("Could not find the thread injection point in the template"))?;
+        .context("Could not find the thread injection point in the template")?;
 
     let threads_json = serde_json::to_string_pretty(&threads_data)
         .context("Failed to serialize threads data to JSON")?;