Misc fixes after FailureDetectorPuppet and abstraction review

- Moved FailureDetectorPuppet to its own file in src/test.
- Removed 'phi' method from FailureDetector public API.
- Throwing exception instead of falling back to default if we can't load the custom FD.
- Removed add-connection method in FailureDetectorPuppet.

Signed-off-by: Jonas Bonér <jonas@jonasboner.com>

akka-cluster/src/main/scala/akka/cluster/Cluster.scala

@@ -309,15 +309,14 @@ object Cluster extends ExtensionId[Cluster] with ExtensionIdProvider {
   override def createExtension(system: ExtendedActorSystem): Cluster = {
     val clusterSettings = new ClusterSettings(system.settings.config, system.name)
 
-    def createDefaultFD() = new AccrualFailureDetector(system, clusterSettings)
     val failureDetector = clusterSettings.FailureDetectorImplementationClass match {
-      case None ⇒ createDefaultFD()
-      case Some(fqcn) ⇒ system.dynamicAccess.createInstanceFor[FailureDetector](fqcn, Seq((classOf[ActorSystem], system), (classOf[ClusterSettings], clusterSettings))) match {
-        case Right(fd) ⇒ fd
-        case Left(e) ⇒
-          system.log.error(e, "Could not create custom failure detector - falling back to default")
-          createDefaultFD()
-      }
+      case None ⇒ new AccrualFailureDetector(system, clusterSettings)
+      case Some(fqcn) ⇒
+        system.dynamicAccess.createInstanceFor[FailureDetector](
+          fqcn, Seq((classOf[ActorSystem], system), (classOf[ClusterSettings], clusterSettings))) match {
+            case Right(fd) ⇒ fd
+            case Left(e)   ⇒ throw new ConfigurationException("Could not create custom failure detector [" + fqcn + "] due to:" + e.toString)
+          }
     }
 
     new Cluster(system, failureDetector)