title: akka.actor.ActorLogging does not log the stack trace of exception by logback
body: <p>I am using <code>Logback + SLF4J</code> to do logging for those actors with trait of <code>akka.actor.ActorLogging</code>. However, when I do the code <code>log.error("Error occur!", e)</code>, the stack trace of the exception <code>e</code> is not logged, but only print a line of <code>Error occur!  WARNING arguments left: 1</code>. I wonder why and how to print the stack trace in the log file. Thank you. The following is my <code>logback.groovy</code> file configuration.</p>
  
  <pre><code>appender("FILE", RollingFileAppender) {
    file = "./logs/logd.txt"
    append = true
    rollingPolicy(TimeBasedRollingPolicy) {
      fileNamePattern = "./logs/logd.%d{yyyy-MM-dd}.log"
      maxHistory = 30
    }
    encoder(PatternLayoutEncoder) {
      pattern = "%date{ISO8601} [%thread] %-5level %logger{36} %X{sourceThread} - %msg%n"
    }
  }
  root(DEBUG, ["FILE"])
  </code></pre>