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/prettier/src/prettier.rs

@@ -1,4 +1,4 @@
-use anyhow::{Context as _, anyhow};
+use anyhow::Context as _;
 use collections::{HashMap, HashSet};
 use fs::Fs;
 use gpui::{AsyncApp, Entity};
@@ -421,7 +421,7 @@ impl Prettier {
                             prettier_parser = prettier_parser.or_else(|| buffer_language.and_then(|language| language.prettier_parser_name()));
                             if prettier_parser.is_none() {
                                 log::error!("Formatting unsaved file with prettier failed. No prettier parser configured for language {buffer_language:?}");
-                                return Err(anyhow!("Cannot determine prettier parser for unsaved file"));
+                                anyhow::bail!("Cannot determine prettier parser for unsaved file");
                             }
 
                         }