fix(core) Include causes when converting anyhow errors to JS exceptions (#16397)

When an op returns an `anyhow` error with a cause (usually added using
the `.context()` method), the `Error` thrown into JavaScript contains
only the message of the outernmost error in the chain.

This PR simply changes the formatting of `anyhow::Error` from `"{}"` to
`"{:#}"`:

This significantly improves errors for code that embeds Deno and defines
custom ops. For example, in
[chiselstrike/chiselstrike](https://github.com/chiselstrike/chiselstrike),
this PR improves an error message like

```
Error: could not plan migration
```

to

```
Error: could not plan migration: could not migrate table for entity "E": could not add column for field "title": the field does not have a default value
```

core/error.rs

@@ -105,7 +105,7 @@ pub fn to_v8_error<'a>(
   let cb = cb.open(scope);
   let this = v8::undefined(scope).into();
   let class = v8::String::new(scope, get_class(error)).unwrap();
-  let message = v8::String::new(scope, &error.to_string()).unwrap();
+  let message = v8::String::new(scope, &format!("{:#}", error)).unwrap();
   let mut args = vec![class.into(), message.into()];
   if let Some(code) = crate::error_codes::get_error_code(error) {
     args.push(v8::String::new(scope, code).unwrap().into());

core/ops.rs

@@ -133,7 +133,7 @@ impl OpError {
   pub fn new(get_class: GetErrorClassFn, err: Error) -> Self {
     Self {
       class_name: (get_class)(&err),
-      message: err.to_string(),
+      message: format!("{:#}", err),
       code: crate::error_codes::get_error_code(&err),
     }
   }

core/runtime.rs

@@ -3094,6 +3094,73 @@ main();
     })
   }
 
+  #[test]
+  fn test_error_context() {
+    use anyhow::anyhow;
+
+    #[op]
+    fn op_err_sync() -> Result<(), Error> {
+      Err(anyhow!("original sync error").context("higher-level sync error"))
+    }
+
+    #[op]
+    async fn op_err_async() -> Result<(), Error> {
+      Err(anyhow!("original async error").context("higher-level async error"))
+    }
+
+    run_in_task(|cx| {
+      let ext = Extension::builder()
+        .ops(vec![op_err_sync::decl(), op_err_async::decl()])
+        .build();
+      let mut runtime = JsRuntime::new(RuntimeOptions {
+        extensions: vec![ext],
+        ..Default::default()
+      });
+
+      runtime
+        .execute_script(
+          "test_error_context_sync.js",
+          r#"
+let errMessage;
+try {
+  Deno.core.ops.op_err_sync();
+} catch (err) {
+  errMessage = err.message;
+}
+if (errMessage !== "higher-level sync error: original sync error") {
+  throw new Error("unexpected error message from op_err_sync: " + errMessage);
+}
+"#,
+        )
+        .unwrap();
+
+      let promise = runtime
+        .execute_script(
+          "test_error_context_async.js",
+          r#"
+(async () => {
+  let errMessage;
+  try {
+    await Deno.core.opAsync("op_err_async");
+  } catch (err) {
+    errMessage = err.message;
+  }
+  if (errMessage !== "higher-level async error: original async error") {
+    throw new Error("unexpected error message from op_err_async: " + errMessage);
+  }
+})()
+"#,
+        )
+        .unwrap();
+
+      match runtime.poll_value(&promise, cx) {
+        Poll::Ready(Ok(_)) => {}
+        Poll::Ready(Err(err)) => panic!("{:?}", err),
+        _ => panic!(),
+      }
+    })
+  }
+
   #[test]
   fn test_pump_message_loop() {
     run_in_task(|cx| {