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/repl/src/notebook/notebook_ui.rs

@@ -565,7 +565,7 @@ impl project::ProjectItem for NotebookItem {
         project: &Entity<Project>,
         path: &ProjectPath,
         cx: &mut App,
-    ) -> Option<Task<gpui::Result<Entity<Self>>>> {
+    ) -> Option<Task<anyhow::Result<Entity<Self>>>> {
         let path = path.clone();
         let project = project.clone();
         let fs = project.read(cx).fs().clone();
@@ -575,7 +575,7 @@ impl project::ProjectItem for NotebookItem {
             Some(cx.spawn(async move |cx| {
                 let abs_path = project
                     .read_with(cx, |project, cx| project.absolute_path(&path, cx))?
-                    .ok_or_else(|| anyhow::anyhow!("Failed to find the absolute path"))?;
+                    .with_context(|| format!("finding the absolute path of {path:?}"))?;
 
                 // todo: watch for changes to the file
                 let file_content = fs.load(&abs_path.as_path()).await?;