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/assistant_tools/src/copy_path_tool.rs

@@ -1,5 +1,5 @@
 use crate::schema::json_schema_for;
-use anyhow::{Result, anyhow};
+use anyhow::{Context as _, Result, anyhow};
 use assistant_tool::{ActionLog, Tool, ToolResult};
 use gpui::AnyWindowHandle;
 use gpui::{App, AppContext, Entity, Task};
@@ -107,17 +107,13 @@ impl Tool for CopyPathTool {
         });
 
         cx.background_spawn(async move {
-            match copy_task.await {
-                Ok(_) => Ok(
-                    format!("Copied {} to {}", input.source_path, input.destination_path).into(),
-                ),
-                Err(err) => Err(anyhow!(
-                    "Failed to copy {} to {}: {}",
-                    input.source_path,
-                    input.destination_path,
-                    err
-                )),
-            }
+            let _ = copy_task.await.with_context(|| {
+                format!(
+                    "Copying {} to {}",
+                    input.source_path, input.destination_path
+                )
+            })?;
+            Ok(format!("Copied {} to {}", input.source_path, input.destination_path).into())
         })
         .into()
     }