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/debugger_ui/src/tests.rs

@@ -1,6 +1,6 @@
 use std::sync::Arc;
 
-use anyhow::{Result, anyhow};
+use anyhow::{Context as _, Result};
 use dap::adapters::DebugTaskDefinition;
 use dap::{DebugRequest, client::DebugAdapterClient};
 use gpui::{Entity, TestAppContext, WindowHandle};
@@ -125,7 +125,7 @@ pub fn start_debug_session_with<T: Fn(&Arc<DebugAdapterClient>) + 'static>(
             .and_then(|panel| panel.read(cx).active_session())
             .map(|session| session.read(cx).running_state().read(cx).session())
             .cloned()
-            .ok_or_else(|| anyhow!("Failed to get active session"))
+            .context("Failed to get active session")
     })??;
 
     Ok(session)