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/gpui/examples/image_loading.rs

@@ -1,6 +1,5 @@
 use std::{path::Path, sync::Arc, time::Duration};
 
-use anyhow::anyhow;
 use gpui::{
     Animation, AnimationExt, App, Application, Asset, AssetLogger, AssetSource, Bounds, Context,
     Hsla, ImageAssetLoader, ImageCacheError, ImgResourceLoader, LOADING_DELAY, Length, Pixels,
@@ -57,7 +56,7 @@ impl Asset for LoadImageWithParameters {
             timer.await;
             if parameters.fail {
                 log::error!("Intentionally failed to load image");
-                Err(anyhow!("Failed to load image").into())
+                Err(anyhow::anyhow!("Failed to load image").into())
             } else {
                 data.await
             }