Parsing alternative XML literals in Scala

I'm working in Scala and need to distinguish between XML messages that represent either success or failure. The information I've found provides lots of insight into pulling apart a known XML fragment but not where you're unsure of which fragment you have.

Here are the two possible messages:

val success = XML.loadString("""<cas:serviceResponse xmlns:cas='http://ift.tt/1jNd6ZE'>
                <cas:authenticationSuccess>
                   <cas:user>bwbecker</cas:user>
                </cas:authenticationSuccess>
              </cas:serviceResponse>""")

val failure = XML.loadString("""<cas:serviceResponse xmlns:cas='http://ift.tt/1jNd6ZE'>
                <cas:authenticationFailure code='INVALID_REQUEST'>
                  &#039;service&#039; and &#039;ticket&#039; parameters are both required
                </cas:authenticationFailure>
              </cas:serviceResponse>""")

The following code does what I want (I'll eventually return an Either of case classes, but this is fine for playing...):

def parse(response: NodeSeq):Either[String, String] = {

(response \ "authenticationSuccess").headOption
  .flatMap(succ => (succ \ "user").headOption)
  .map(usr => Right(usr.text))
  .getOrElse((response \ "authenticationFailure").headOption
    .map{fail => 
        val code = fail \ "@code"
        val msg = fail.text
        Left(s"Failure: ${code} ${msg}")
    }
    .getOrElse(Left("Really Botched"))
  )
}

However, I find this really hard to code and read. Is there a better way? What if I had five different messages to distinguish?

I tried matchers, but got discouraged with the arcane syntax for XML (the cas: namespace seems to complicate things).

Any guidance for improving my code?