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/agent/src/buffer_codegen.rs

@@ -1,7 +1,7 @@
 use crate::context::ContextLoadResult;
 use crate::inline_prompt_editor::CodegenStatus;
 use crate::{context::load_context, context_store::ContextStore};
-use anyhow::Result;
+use anyhow::{Context as _, Result};
 use assistant_settings::AssistantSettings;
 use client::telemetry::Telemetry;
 use collections::HashSet;
@@ -419,16 +419,16 @@ impl CodegenAlternative {
             if start_buffer.remote_id() == end_buffer.remote_id() {
                 (start_buffer.clone(), start_buffer_offset..end_buffer_offset)
             } else {
-                return Err(anyhow::anyhow!("invalid transformation range"));
+                anyhow::bail!("invalid transformation range");
             }
         } else {
-            return Err(anyhow::anyhow!("invalid transformation range"));
+            anyhow::bail!("invalid transformation range");
         };
 
         let prompt = self
             .builder
             .generate_inline_transformation_prompt(user_prompt, language_name, buffer, range)
-            .map_err(|e| anyhow::anyhow!("Failed to generate content prompt: {}", e))?;
+            .context("generating content prompt")?;
 
         let context_task = self.context_store.as_ref().map(|context_store| {
             if let Some(project) = self.project.upgrade() {