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/embedding_index.rs

@@ -3,7 +3,7 @@ use crate::{
     embedding::{Embedding, EmbeddingProvider, TextToEmbed},
     indexing::{IndexingEntryHandle, IndexingEntrySet},
 };
-use anyhow::{Context as _, Result, anyhow};
+use anyhow::{Context as _, Result};
 use collections::Bound;
 use feature_flags::FeatureFlagAppExt;
 use fs::Fs;
@@ -422,7 +422,7 @@ impl EmbeddingIndex {
                 .context("failed to create read transaction")?;
             Ok(db
                 .get(&tx, &db_key_for_path(&path))?
-                .ok_or_else(|| anyhow!("no such path"))?
+                .context("no such path")?
                 .chunks
                 .clone())
         })