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/image_viewer/src/image_viewer.rs

@@ -222,11 +222,11 @@ impl SerializableItem for ImageView {
         item_id: ItemId,
         window: &mut Window,
         cx: &mut App,
-    ) -> Task<gpui::Result<Entity<Self>>> {
+    ) -> Task<anyhow::Result<Entity<Self>>> {
         window.spawn(cx, async move |cx| {
             let image_path = IMAGE_VIEWER
                 .get_image_path(item_id, workspace_id)?
-                .ok_or_else(|| anyhow::anyhow!("No image path found"))?;
+                .context("No image path found")?;
 
             let (worktree, relative_path) = project
                 .update(cx, |project, cx| {
@@ -256,7 +256,7 @@ impl SerializableItem for ImageView {
         alive_items: Vec<ItemId>,
         _window: &mut Window,
         cx: &mut App,
-    ) -> Task<gpui::Result<()>> {
+    ) -> Task<anyhow::Result<()>> {
         delete_unloaded_items(
             alive_items,
             workspace_id,
@@ -273,7 +273,7 @@ impl SerializableItem for ImageView {
         _closing: bool,
         _window: &mut Window,
         cx: &mut Context<Self>,
-    ) -> Option<Task<gpui::Result<()>>> {
+    ) -> Option<Task<anyhow::Result<()>>> {
         let workspace_id = workspace.database_id()?;
         let image_path = self.image_item.read(cx).abs_path(cx)?;
 

crates/image_viewer/src/image_viewer_settings.rs

@@ -28,10 +28,7 @@ impl Settings for ImageViewerSettings {
 
     type FileContent = Self;
 
-    fn load(
-        sources: SettingsSources<Self::FileContent>,
-        _: &mut App,
-    ) -> Result<Self, anyhow::Error> {
+    fn load(sources: SettingsSources<Self::FileContent>, _: &mut App) -> anyhow::Result<Self> {
         SettingsSources::<Self::FileContent>::json_merge_with(
             [sources.default]
                 .into_iter()