Make Logback throw exception on ERROR level log events

When running unit tests, I'd like to fail any tests during which ERROR level message is logged. What would be the easiest way to achieve this using SLF4J/Logback? I'd like to avoid writing my own ILoggerFactory implementation.

I tried writing a custom Appender, but I cannot propagate exceptions through the code that's calling the Appender, all exceptions from Appender get caught there.