Fix error recovery when serializing unknown structs in JS view payload

lib/livebook_web/channels/js_view_channel.ex

@@ -105,14 +105,17 @@ defmodule LivebookWeb.JSViewChannel do
   defp run_safely(fun) do
     try do
       {:ok, fun.()}
-    catch
-      :error, %Protocol.UndefinedError{protocol: Jason.Encoder, value: value} ->
+    rescue
+      error ->
+        case error do
+          %Protocol.UndefinedError{protocol: Jason.Encoder, value: value} ->
             {:error, "value #{inspect(value)} is not JSON-serializable, use another data type"}
 
-      :error, error ->
+          error ->
             {:error, Exception.message(error)}
         end
     end
+  end
 
   defp fastlane_encoder({:event, event, payload, %{ref: ref}}) do
     run_safely(fn ->