Gracefully handle the absence of NSIS

[savornicesei]

There are 2 situations where NSIS does not exists on the build system:

1. it's not a Windows machine
2. it was not installed

If NSIS is not available we should handle it gracefully in ccnet-packaging.inc.build, targets:

packageCCTray
packageCCTools
packageCCNET

I'm thinking if not on Windows it should display something like "CruiseControl.NET windows installers can only be created on Windows". But if the script runs on Windows but nsis.executable is empty / does not exist then it should fail the build.

Original discussion on PR #301