Adding DiracKernel print statements.

Adding back proc_id aux var.

FIXME: Error out from non-local cached element case.

It appears that updateMesh() gets called before Dirac points are
added, so that would be too late.

It also appears that PointLocators on different processors which are
searching for points in a DisplacedMesh can come up with different
Elements.  Working on a patch for PointLocator that makes it run only
over local elements...