ENH: Add ITK_FORBID_DOWNLOADS option. ITK-3239

Suggestion by Gert Wollny:

  When packaging the software, e.g. for Debian, one can not assume that the
  build machine is connected to the Internet. In order to make it easy to ensure
  that no downloads are required during the build, a CMake option should be added
  that forbids this and issues errors about missing third party tools/packages.
  This is also nice for people who download the software at a place with a
  reasonable Internet connection, but want to build it elsewhere/later.

A new advanced option, ITK_FORBID_DOWNLOADS is added.  If on, all
ExternalProject_Add calls will send a CMake error.  Also, skip the remote
URL's for ExternalData.

Change-Id: I7863bf87f1d6fd3f07939e3d580e3ae864076f22
COMP: return will never be executed after exception thrown.

Change-Id: I1e6bbb7fb7780de546b7c5090f697ba55bda5b70
Merge topic 'forbid-download'

e5e0b3d2 ENH: Add ITK_FORBID_DOWNLOADS option. ITK-3239
Merge topic 'return-exception'

de48d07b COMP: return will never be executed after exception thrown.