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/eval/src/example.rs

@@ -177,12 +177,10 @@ impl ExampleContext {
 
     fn log_assertion<T>(&mut self, result: Result<T>, message: String) -> Result<T> {
         if let Some(max) = self.meta.max_assertions {
-            if self.assertions.run_count() > max {
-                return Err(anyhow!(
-                    "More assertions were run than the stated max_assertions of {}",
-                    max
-                ));
-            }
+            anyhow::ensure!(
+                self.assertions.run_count() <= max,
+                "More assertions were run than the stated max_assertions of {max}"
+            );
         }
 
         self.assertions.ran.push(RanAssertion {
@@ -319,7 +317,7 @@ impl ExampleContext {
                     }
                 }
                 _ = self.app.background_executor().timer(THREAD_EVENT_TIMEOUT).fuse() => {
-                    return Err(anyhow!("Agentic loop stalled - waited {:?} without any events", THREAD_EVENT_TIMEOUT));
+                    anyhow::bail!("Agentic loop stalled - waited {THREAD_EVENT_TIMEOUT:?} without any events");
                 }
             }
         }