No more taxonomy widget WSOD #3430

1. Box form element validates that its a positive integer.  Yay generic validation funcitons.
2. Range is only applied if it's a positive int.
3. The bail out when term list is empty check happens after the range is applied (probably redundant with 2, but no reason not to move it)
Merge pull request #3431 from openscholar/3430-taxonomy_box_wsod

No more taxonomy widget WSOD #3430, code looks good given that https://github.com/openscholar/openscholar/pull/3431#issuecomment-21949063 holds true