core: Tone down MessageExecutionError

The display impl of the error prints a stack trace, which is not
particularly helpful given it is usually a user error.

Use the alternate format (`{:#}`) instead, which makes anyhow display a
single line and no backtrace.

Author: kim

crates/core/src/client/message_handlers.rs

@@ -141,7 +141,7 @@ pub async fn handle(client: &ClientConnection, message: DataMessage, timer: Inst
 }
 
 #[derive(thiserror::Error, Debug)]
-#[error("error executing message (reducer: {reducer:?}) (err: {err:?})")]
+#[error("error executing message (reducer: {reducer:?}) (err: {err:#})")]
 pub struct MessageExecutionError {
     pub reducer: Option<Box<str>>,
     pub reducer_id: Option<ReducerId>,