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/rpc/src/message_stream.rs

@@ -2,7 +2,6 @@
 
 pub use ::proto::*;
 
-use anyhow::anyhow;
 use async_tungstenite::tungstenite::Message as WebSocketMessage;
 use futures::{SinkExt as _, StreamExt as _};
 use proto::Message as _;
@@ -40,7 +39,7 @@ impl<S> MessageStream<S>
 where
     S: futures::Sink<WebSocketMessage, Error = anyhow::Error> + Unpin,
 {
-    pub async fn write(&mut self, message: Message) -> Result<(), anyhow::Error> {
+    pub async fn write(&mut self, message: Message) -> anyhow::Result<()> {
         #[cfg(any(test, feature = "test-support"))]
         const COMPRESSION_LEVEL: i32 = -7;
 
@@ -81,9 +80,9 @@ where
 
 impl<S> MessageStream<S>
 where
-    S: futures::Stream<Item = Result<WebSocketMessage, anyhow::Error>> + Unpin,
+    S: futures::Stream<Item = anyhow::Result<WebSocketMessage>> + Unpin,
 {
-    pub async fn read(&mut self) -> Result<(Message, Instant), anyhow::Error> {
+    pub async fn read(&mut self) -> anyhow::Result<(Message, Instant)> {
         while let Some(bytes) = self.stream.next().await {
             let received_at = Instant::now();
             match bytes? {
@@ -102,7 +101,7 @@ where
                 _ => {}
             }
         }
-        Err(anyhow!("connection closed"))
+        anyhow::bail!("connection closed");
     }
 }
 
@@ -113,7 +112,7 @@ mod tests {
     #[gpui::test]
     async fn test_buffer_size() {
         let (tx, rx) = futures::channel::mpsc::unbounded();
-        let mut sink = MessageStream::new(tx.sink_map_err(|_| anyhow!("")));
+        let mut sink = MessageStream::new(tx.sink_map_err(|_| anyhow::anyhow!("")));
         sink.write(Message::Envelope(Envelope {
             payload: Some(envelope::Payload::UpdateWorktree(UpdateWorktree {
                 root_name: "abcdefg".repeat(10),