Change log message to use a message and arguments instead of a formatting pattern

[stefansaasen]

I'm seeing the following error in the river logs:

{"time":"2024-06-10T15:02:54.573951+02:00","level":"ERROR","msg":"Error waiting on async completer: %s","!BADKEY":"context deadline exceeded"}

Change the log message to add the error as a log argument (like in the rest of the code base).