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/session.rs

@@ -6,6 +6,7 @@ use crate::{
     kernels::{Kernel, KernelSpecification, NativeRunningKernel},
     outputs::{ExecutionStatus, ExecutionView},
 };
+use anyhow::Context as _;
 use collections::{HashMap, HashSet};
 use editor::{
     Anchor, AnchorRangeExt as _, Editor, MultiBuffer, ToPoint,
@@ -57,13 +58,8 @@ impl EditorBlock {
         on_close: CloseBlockFn,
         cx: &mut Context<Session>,
     ) -> anyhow::Result<Self> {
-        let editor = editor
-            .upgrade()
-            .ok_or_else(|| anyhow::anyhow!("editor is not open"))?;
-        let workspace = editor
-            .read(cx)
-            .workspace()
-            .ok_or_else(|| anyhow::anyhow!("workspace dropped"))?;
+        let editor = editor.upgrade().context("editor is not open")?;
+        let workspace = editor.read(cx).workspace().context("workspace dropped")?;
 
         let execution_view = cx.new(|cx| ExecutionView::new(status, workspace.downgrade(), cx));