This avoids unnecessary second message when left blank:
> can't be blank
> is not a number
Ok this is a little confusing. Why is there a separate presence check above, and why is it only for measurable units, when we still require a number for _all_ units? Because, for 'items', we allow a blank value then auto-set it to 1.
I don't know if it's really necessary, but that's how it currently works...
7164cbd0bd