ENH: Improve appveyor CI
ENH: Improve appveyor CI

This commit generally improves CI on AppVeyor.

-Explicitely fail the first time ITK is built (with "exit 1)" to allow for
caching.

-Only build itkTestDriver if it does not exist already.

-Try to build ITKData before RTK and re-build it if network errors occured.

SimonRit/RTK,SimonRit/RTK,SimonRit/RTK,SimonRit/RTK