New reporter message for measure missing VC failing

[samuelchassot]

the new message reports that the measure is missing instead of an empty counterexample.
ex:

[Warning ]  - Result for 'measure missing' VC for leftPad @33:7:
[Warning ] false
[Warning ] LeftPad.scala:33:7:  => INVALID
             def leftPad[T](c: T, n: BigInt, s: List[T]): List[T] = {
                 ^
[Warning ] Measure is missing, cannot check termination