How can I custom the response error message in different situations?

[zemelLeong]

I want to use this err as the response content.

[jebrosen]

This is really close.

First, you'll have to change the return type of delete_document so you can return either a success or error response:

pub async fn delete_document(conn: MysqlDBConn, ids: Json<Vec<i32>>) -> Result<String, String> { ... }

Then, the handler can return a response according to the result of that database operation:

let res = conn.run(...).await;
match res {
    Ok(count) => Ok(format!("Deleted {} records", count)),
    Err(e) => Err(e.to_string()),
}

And, you can change Result<String, String> to whatever you like depending on how that route needs to respond - e.g. Result<Template, Json<MadeUpErrorType>>.