Better error info for "Invalid command invocation"

[exyi]

The FindCommand procedure usually fail because one of DataContext path, validation path or binding id gets wrong, so listing all command on the page sorted by simmilarity would help a lot
command on the page sorted by simmilarity would help a lot. The error also occurs when command inside Repeater is invoked but the collection is not populated, so stating "Nothing was found in found inside specified DataContext, check if ViewModel is populated" could help.

[exyi]

resolved by #386, but if you'd have more ideas to make this error message more useful, feel free to comment.