Report error on 0-class, 0-property vocab imports

[zerocrates]

This would avoid having to delete the empty vocab after a failed import.

Also, since this is usually caused by an incorrect namespace URI, we could mention that in the error specifically.

[zerocrates]

I do think something along the lines of "Are you sure you used the correct namespace URI?" is prudent to include in the error message itself since it seems to be by far the most common cause of this issue.