Adding an expect_err test that demonstrates the error handling when
you try to cache a point with a "unique" ID that's already taken.

Refs #2364.
Adding Dirac point reverse cache id lookup as requested by @WilkAndy

Refs #2364.
Update the CachingPointSource test to look up the current point's ID.

Also renamed the output and gold files to avoid clashing with an expect_err
test in the same directory that used the same filename.

Refs #2364.
Use DiracKernel caching and currentPointCachedID()
in RichardsBorehole and RichardsPolyLineSink.

Refs #2364.