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/snippet/src/snippet.rs

@@ -1,4 +1,4 @@
-use anyhow::{Context as _, Result, anyhow};
+use anyhow::{Context as _, Result};
 use smallvec::SmallVec;
 use std::{collections::BTreeMap, ops::Range};
 
@@ -114,7 +114,7 @@ fn parse_tabstop<'a>(
         if source.starts_with('}') {
             source = &source[1..];
         } else {
-            return Err(anyhow!("expected a closing brace"));
+            anyhow::bail!("expected a closing brace");
         }
     } else {
         let (index, rest) = parse_int(source)?;
@@ -137,9 +137,7 @@ fn parse_int(source: &str) -> Result<(usize, &str)> {
     let len = source
         .find(|c: char| !c.is_ascii_digit())
         .unwrap_or(source.len());
-    if len == 0 {
-        return Err(anyhow!("expected an integer"));
-    }
+    anyhow::ensure!(len > 0, "expected an integer");
     let (prefix, suffix) = source.split_at(len);
     Ok((prefix.parse()?, suffix))
 }
@@ -180,11 +178,10 @@ fn parse_choices<'a>(
             Some(_) => {
                 let chunk_end = source.find([',', '|', '\\']);
 
-                if chunk_end.is_none() {
-                    return Err(anyhow!(
-                        "Placeholder choice doesn't contain closing pipe-character '|'"
-                    ));
-                }
+                anyhow::ensure!(
+                    chunk_end.is_some(),
+                    "Placeholder choice doesn't contain closing pipe-character '|'"
+                );
 
                 let (chunk, rest) = source.split_at(chunk_end.unwrap());