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/terminal_view/src/terminal_view.rs

@@ -1668,7 +1668,7 @@ impl SerializableItem for TerminalView {
         alive_items: Vec<workspace::ItemId>,
         _window: &mut Window,
         cx: &mut App,
-    ) -> Task<gpui::Result<()>> {
+    ) -> Task<anyhow::Result<()>> {
         delete_unloaded_items(alive_items, workspace_id, "terminals", &TERMINAL_DB, cx)
     }
 
@@ -1679,7 +1679,7 @@ impl SerializableItem for TerminalView {
         _closing: bool,
         _: &mut Window,
         cx: &mut Context<Self>,
-    ) -> Option<Task<gpui::Result<()>>> {
+    ) -> Option<Task<anyhow::Result<()>>> {
         let terminal = self.terminal().read(cx);
         if terminal.task().is_some() {
             return None;