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/semantic_index/src/semantic_index.rs

@@ -264,7 +264,6 @@ impl Drop for SemanticDb {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use anyhow::anyhow;
     use chunking::Chunk;
     use embedding_index::{ChunkedFile, EmbeddingIndex};
     use feature_flags::FeatureFlagAppExt;
@@ -446,15 +445,15 @@ mod tests {
         cx.executor().allow_parking();
 
         let provider = Arc::new(TestEmbeddingProvider::new(3, |text| {
-            if text.contains('g') {
-                Err(anyhow!("cannot embed text containing a 'g' character"))
-            } else {
-                Ok(Embedding::new(
-                    ('a'..='z')
-                        .map(|char| text.chars().filter(|c| *c == char).count() as f32)
-                        .collect(),
-                ))
-            }
+            anyhow::ensure!(
+                !text.contains('g'),
+                "cannot embed text containing a 'g' character"
+            );
+            Ok(Embedding::new(
+                ('a'..='z')
+                    .map(|char| text.chars().filter(|c| *c == char).count() as f32)
+                    .collect(),
+            ))
         }));
 
         let (indexing_progress_tx, _) = channel::unbounded();