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/ids.rs

@@ -1,4 +1,4 @@
-use anyhow::{Result, anyhow};
+use anyhow::{Context as _, Result};
 use std::fs;
 use std::path::{Path, PathBuf};
 use uuid::Uuid;
@@ -11,7 +11,7 @@ pub fn get_or_create_id(path: &Path) -> Result<String> {
         }
     }
     let new_id = Uuid::new_v4().to_string();
-    fs::create_dir_all(path.parent().ok_or_else(|| anyhow!("invalid id path"))?)?;
+    fs::create_dir_all(path.parent().context("invalid id path")?)?;
     fs::write(path, &new_id)?;
     Ok(new_id)
 }