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/project/src/debugger/session.rs

@@ -12,7 +12,7 @@ use super::dap_command::{
     TerminateThreadsCommand, ThreadsCommand, VariablesCommand,
 };
 use super::dap_store::DapStore;
-use anyhow::{Result, anyhow};
+use anyhow::{Context as _, Result, anyhow};
 use collections::{HashMap, HashSet, IndexMap, IndexSet};
 use dap::adapters::{DebugAdapterBinary, DebugAdapterName};
 use dap::messages::Response;
@@ -487,8 +487,7 @@ impl Mode {
         match self {
             Mode::Running(debug_adapter_client) => debug_adapter_client.request(request),
             Mode::Building => Task::ready(Err(anyhow!(
-                "no adapter running to send request: {:?}",
-                request
+                "no adapter running to send request: {request:?}"
             ))),
         }
     }
@@ -1736,7 +1735,7 @@ impl Session {
             anyhow::Ok(
                 task.await
                     .map(|response| response.targets)
-                    .ok_or_else(|| anyhow!("failed to fetch completions"))?,
+                    .context("failed to fetch completions")?,
             )
         })
     }