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/task/src/debug_format.rs

@@ -1,4 +1,4 @@
-use anyhow::Result;
+use anyhow::{Context as _, Result};
 use collections::FxHashMap;
 use gpui::SharedString;
 use schemars::{JsonSchema, r#gen::SchemaSettings};
@@ -147,9 +147,7 @@ impl DebugRequest {
     }
 
     pub fn from_proto(val: proto::DebugRequest) -> Result<DebugRequest> {
-        let request = val
-            .request
-            .ok_or_else(|| anyhow::anyhow!("Missing debug request"))?;
+        let request = val.request.context("Missing debug request")?;
         match request {
             proto::debug_request::Request::DebugLaunchRequest(proto::DebugLaunchRequest {
                 program,