Intentionally cripple Box's .get method for Lift 3.

Box.get now returns Nothing, which should break anyone
attempting to use its return type for anything, including tests.

Also fix a Box test that relied on .get.

core/common/src/main/scala/net/liftweb/common/Box.scala

@@ -255,15 +255,12 @@ sealed abstract class Box[+A] extends Product with Serializable{
   final def open_! : A = openOrThrowException("Legacy method implementation")
   */
   /**
-   * Return the value contained in this Box if it is Full; throw an
-   * exception otherwise.  Please use openOrThrowException instead. In
-   * the past, this method triggered an implicit conversion to Option
-   * and could throw an unintended NullPointerException. That is no longer
-   * the case, and in Lift 3 this method will be changed to return
-   * a useless type so that the compiler will break attempts to use it.
-   */
-  @deprecated("use map/flatMap/foreach if possible, or openOrThrowException if you must", "2.6")
-  final def get: A = open_!
+   * Exists to avoid the implicit conversion from Box to Option. Opening a Box
+   * unsafely should be done using openOrThrowException.
+   */
+  final def get: Nothing = {
+    throw new Exception("Attempted to open a Box incorrectly. Please use openOrThrowException.")
+  }
 
   /**
    * Return the value contained in this Box if it is Full;

core/common/src/test/scala/net/liftweb/common/BoxSpec.scala

@@ -79,11 +79,6 @@ class BoxSpec extends Specification with ScalaCheck with BoxGenerator {
     "be defined from some legacy code (possibly passing null values). If the passed value is null, an Empty is returned" in {
       Box.legacyNullTest(null) must_== Empty
     }
-
-    "have get defined in a way compatible with Option" in {
-      Full(1).get must_== 1
-      (Empty: Box[Int]).get must throwA[NullPointerException]
-    }
   }
 
   "A Box" should {
@@ -299,7 +294,7 @@ class BoxSpec extends Specification with ScalaCheck with BoxGenerator {
   "A Failure is an Empty Box which" can {
     "return its cause as an exception" in {
       case class LiftException(m: String) extends Exception
-      Failure("error", Full(new LiftException("broken")), Empty).exception.get must_== new LiftException("broken")
+      Failure("error", Full(new LiftException("broken")), Empty).exception must_== Full(new LiftException("broken"))
     }
     "return a chained list of causes" in {
       Failure("error",