Support to write error messages directly to JSON

firmware/src/error.rs

@@ -1,7 +1,10 @@
+use crate::json::{self, ToJson};
 use crate::user::UserId;
 use crate::{display, nfc, vereinsflieger};
+use alloc::string::ToString;
 use core::fmt;
 use core::future::Future;
+use embedded_io_async::Write;
 
 /// Main error type
 #[derive(Debug)]
@@ -126,3 +129,12 @@ impl fmt::Display for ErrorKind {
         }
     }
 }
+
+impl ToJson for ErrorKind {
+    async fn to_json<W: Write>(
+        &self,
+        json: &mut json::Writer<W>,
+    ) -> Result<(), json::Error<W::Error>> {
+        json.write(self.to_string()).await
+    }
+}