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/theme_importer/src/assets.rs

@@ -1,6 +1,6 @@
 use std::borrow::Cow;
 
-use anyhow::{Result, anyhow};
+use anyhow::{Context as _, Result};
 use gpui::{AssetSource, SharedString};
 use rust_embed::RustEmbed;
 
@@ -14,7 +14,7 @@ impl AssetSource for Assets {
     fn load(&self, path: &str) -> Result<Option<Cow<'static, [u8]>>> {
         Self::get(path)
             .map(|f| f.data)
-            .ok_or_else(|| anyhow!("could not find asset at path \"{}\"", path))
+            .with_context(|| format!("could not find asset at path {path:?}"))
             .map(Some)
     }