Added robust amount validation to trading form

It checks that a form is filled out and if so, check if there is an error. (Yeah, it checks for errors even if form isn't filled out, but it shouldn't be a performance issue).
I'm comparing for strings right here to see if it is filled. I'm not checking to see if stuff is filled out validly.

I probably shouldn't check for 0 because that was when we had the placeholder.