COMP: Ensure FindDCMTK module works with and without DCMTKConfig.cmake file

In the case the provided DCMTK build or install tree doesn't have a
DCMTKConfig.cmake file, the first call to "find_package" should fail
quietly so that it doesn't prevent the rest of discovery process from working
as expected.

See #329
COMP: Make FindDCMTK more verbose

It will display the following text:

-- Trying to find DCMTK expecting DCMTKConfig.cmake
-- Trying to find DCMTK expecting DCMTKConfig.cmake - failed
-- Trying to find DCMTK relying on FindDCMTK.cmake
-- Trying to find DCMTK relying on FindDCMTK.cmake - ok