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/gpui/src/keymap/context.rs

@@ -1,5 +1,5 @@
 use crate::SharedString;
-use anyhow::{Result, anyhow};
+use anyhow::{Context as _, Result};
 use std::fmt;
 
 /// A datastructure for resolving whether an action should be dispatched
@@ -243,7 +243,7 @@ impl KeyBindingContextPredicate {
         let source = skip_whitespace(source);
         let (predicate, rest) = Self::parse_expr(source, 0)?;
         if let Some(next) = rest.chars().next() {
-            Err(anyhow!("unexpected character '{next:?}'"))
+            anyhow::bail!("unexpected character '{next:?}'");
         } else {
             Ok(predicate)
         }
@@ -329,20 +329,14 @@ impl KeyBindingContextPredicate {
     }
 
     fn parse_primary(mut source: &str) -> anyhow::Result<(Self, &str)> {
-        let next = source
-            .chars()
-            .next()
-            .ok_or_else(|| anyhow!("unexpected end"))?;
+        let next = source.chars().next().context("unexpected end")?;
         match next {
             '(' => {
                 source = skip_whitespace(&source[1..]);
                 let (predicate, rest) = Self::parse_expr(source, 0)?;
-                if let Some(stripped) = rest.strip_prefix(')') {
-                    source = skip_whitespace(stripped);
-                    Ok((predicate, source))
-                } else {
-                    Err(anyhow!("expected a ')'"))
-                }
+                let stripped = rest.strip_prefix(')').context("expected a ')'")?;
+                source = skip_whitespace(stripped);
+                Ok((predicate, source))
             }
             '!' => {
                 let source = skip_whitespace(&source[1..]);
@@ -368,7 +362,7 @@ impl KeyBindingContextPredicate {
                     source,
                 ))
             }
-            _ => Err(anyhow!("unexpected character '{next:?}'")),
+            _ => anyhow::bail!("unexpected character '{next:?}'"),
         }
     }
 
@@ -388,7 +382,7 @@ impl KeyBindingContextPredicate {
         if let (Self::Identifier(left), Self::Identifier(right)) = (self, other) {
             Ok(Self::Equal(left, right))
         } else {
-            Err(anyhow!("operands of == must be identifiers"))
+            anyhow::bail!("operands of == must be identifiers");
         }
     }
 
@@ -396,7 +390,7 @@ impl KeyBindingContextPredicate {
         if let (Self::Identifier(left), Self::Identifier(right)) = (self, other) {
             Ok(Self::NotEqual(left, right))
         } else {
-            Err(anyhow!("operands of != must be identifiers"))
+            anyhow::bail!("operands of != must be identifiers");
         }
     }
 }