Reject invalid input in int2vectorin.

Reject invalid input in int2vectorin.

Since the int2vector type is intended only for internal use, this patch doesn't
worry about prettifying the error messages, which has the fringe benefit of
avoiding creating additional translatable strings.  For a type intended to be
used by end-users, we would want to do better, but the approach taken here
seems like the correct trade-off for this case.

Caleb Welton