Catch invalid $kref in Netkan

[HebaruSan]

Problem

Currently if you have an invalid $kref:

    "$kref"        : "#/ckan/techman/iscool",

The error message isn't super clear:

$ ../CKAN/_build/netkan.exe NetKAN/KSP-AVC.netkan 
343 [1] ERROR CKAN.CkanModule (null) - KSP-AVC missing required field download
354 [1] FATAL CKAN.NetKAN.Program (null) - KSP-AVC missing required field download

Here it's telling us that it doesn't have a download property, which is indeed caused by the $kref being invalid, but it's rather indirect and not necessarily clear to a new user.

Changes

Now the NetkanValidator has a new validation step that checks whether metadata.Kref.Source is a known valid value (null is also OK). If not, it prints a new error message:

$ ../CKAN/_build/netkan.exe NetKAN/KSP-AVC.netkan 
288 [1] FATAL CKAN.NetKAN.Program (null) - Invalid $kref: #/ckan/techman/iscool

RemoteRef.ToString previously dropped the /ckan/ portion of the original string; #/ckan/techman/iscool became #/techman/iscool. To reduce confusion, this function now preserves the original string.

Fixes #374.