Improve error message in popup validation. Closes #1255
Improve error message in popup validation. Closes #1255

sergey-filipenko/vizabi,sergey-filipenko/vizabi,Gapminder/vizabi,vizabi/vizabi,dab2000/vizabi,Gapminder/vizabi,buchslava/vizabi,dab2000/vizabi,dab2000/vizabi,vizabi/vizabi,buchslava/vizabi,Gapminder/vizabi,buchslava/vizabi,sergey-filipenko/vizabi