Python bindings silently fail if XRT is not found.

[stephenneuendorffer]

If the XRT setup script is not found, then many iron bindings are silently not injected into the iron namespace. The resulting message looks like a python setup problem and is misleading. It would be better to either fail early, or to inject dummy names that fail later with a good error message.

[hunhoffe]

#2692 addresses part of this issue. XRT no longer causes silent failures, but the failures aren't early either and there is not a good failure message. I'll leave this open because it's only been partially addressed.

[hunhoffe]

This was addressed by #2737