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/language/src/syntax_map.rs

@@ -4,6 +4,7 @@ mod syntax_map_tests;
 use crate::{
     Grammar, InjectionConfig, Language, LanguageId, LanguageRegistry, QUERY_CURSORS, with_parser,
 };
+use anyhow::Context as _;
 use collections::HashMap;
 use futures::FutureExt;
 use std::{
@@ -1246,7 +1247,7 @@ fn parse_text(
                 old_tree.as_ref(),
                 None,
             )
-            .ok_or_else(|| anyhow::anyhow!("failed to parse"))
+            .context("failed to parse")
     })
 }