Provide feedback to distinguish between "no matches" and "error in XPath" for XML Data Source pickers

[greystate]

When writing the Options XPath it would be nice to get some feedback, when the supplied XPath is invalid in some way, e.g. if you forget a closing square bracket or similar. Right now, the only clue you get is that on the property using the picker, there are no options showing - which is exactly the same result you get if the XPath is in fact valid, but for some reason there are no matching nodes to pick (for whatever reason).

An erroneous XPath should ideally be caught already when saved, so the developer can fix the Data Type without having to switch tabs to the Content section. (Editors should never see an error like this on the property.)

Being able to specify a "No matches" message (though it's yet another field to fill in) would be great, actually.