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/create_directory_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, Entity, Task};
@@ -86,7 +86,7 @@ impl Tool for CreateDirectoryTool {
                     project.create_entry(project_path.clone(), true, cx)
                 })?
                 .await
-                .map_err(|err| anyhow!("Unable to create directory {destination_path}: {err}"))?;
+                .with_context(|| format!("Creating directory {destination_path}"))?;
 
             Ok(format!("Created directory {destination_path}").into())
         })