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/editor/src/proposed_changes_editor.rs

@@ -355,7 +355,7 @@ impl Item for ProposedChangesEditor {
         project: Entity<Project>,
         window: &mut Window,
         cx: &mut Context<Self>,
-    ) -> Task<gpui::Result<()>> {
+    ) -> Task<anyhow::Result<()>> {
         self.editor.update(cx, |editor, cx| {
             Item::save(editor, format, project, window, cx)
         })
@@ -488,7 +488,7 @@ impl SemanticsProvider for BranchBufferSemanticsProvider {
         buffer: &Entity<Buffer>,
         position: text::Anchor,
         cx: &mut App,
-    ) -> Option<Task<gpui::Result<Vec<project::DocumentHighlight>>>> {
+    ) -> Option<Task<anyhow::Result<Vec<project::DocumentHighlight>>>> {
         let buffer = self.to_base(&buffer, &[position], cx)?;
         self.0.document_highlights(&buffer, position, cx)
     }
@@ -499,7 +499,7 @@ impl SemanticsProvider for BranchBufferSemanticsProvider {
         position: text::Anchor,
         kind: crate::GotoDefinitionKind,
         cx: &mut App,
-    ) -> Option<Task<gpui::Result<Vec<project::LocationLink>>>> {
+    ) -> Option<Task<anyhow::Result<Vec<project::LocationLink>>>> {
         let buffer = self.to_base(&buffer, &[position], cx)?;
         self.0.definitions(&buffer, position, kind, cx)
     }
@@ -509,7 +509,7 @@ impl SemanticsProvider for BranchBufferSemanticsProvider {
         _: &Entity<Buffer>,
         _: text::Anchor,
         _: &mut App,
-    ) -> Option<Task<gpui::Result<Option<Range<text::Anchor>>>>> {
+    ) -> Option<Task<anyhow::Result<Option<Range<text::Anchor>>>>> {
         None
     }
 
@@ -519,7 +519,7 @@ impl SemanticsProvider for BranchBufferSemanticsProvider {
         _: text::Anchor,
         _: String,
         _: &mut App,
-    ) -> Option<Task<gpui::Result<project::ProjectTransaction>>> {
+    ) -> Option<Task<anyhow::Result<project::ProjectTransaction>>> {
         None
     }
 }