intvec a:b with b<0 or b == 0

[seelisch]

reported by Hrn. Gorzel: A more informative error description could be helpful:

2:0;

? int : int failed ? expected int : int ? error occurred in STDIN line 1: 2:0;

2:-2;

? int : int failed ? expected int : int ? error occurred in STDIN line 2: 2:-2;

[hannes]

*hannes: changed to:

2:-2;

? int : int failed ? error occurred in STDIN line 1: 2:-2;

[hannes]

error message fixed