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_context_editor/src/context.rs

@@ -1,7 +1,7 @@
 #[cfg(test)]
 mod context_tests;
 
-use anyhow::{Context as _, Result, anyhow, bail};
+use anyhow::{Context as _, Result, bail};
 use assistant_settings::AssistantSettings;
 use assistant_slash_command::{
     SlashCommandContent, SlashCommandEvent, SlashCommandLine, SlashCommandOutputSection,
@@ -3011,7 +3011,7 @@ impl SavedContext {
         let saved_context_json = serde_json::from_str::<serde_json::Value>(json)?;
         match saved_context_json
             .get("version")
-            .ok_or_else(|| anyhow!("version not found"))?
+            .context("version not found")?
         {
             serde_json::Value::String(version) => match version.as_str() {
                 SavedContext::VERSION => {
@@ -3032,9 +3032,9 @@ impl SavedContext {
                         serde_json::from_value::<SavedContextV0_1_0>(saved_context_json)?;
                     Ok(saved_context.upgrade())
                 }
-                _ => Err(anyhow!("unrecognized saved context version: {}", version)),
+                _ => anyhow::bail!("unrecognized saved context version: {version:?}"),
             },
-            _ => Err(anyhow!("version not found on saved context")),
+            _ => anyhow::bail!("version not found on saved context"),
         }
     }