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).

https://github.com/stellar/stellar-client/issues/891
```canSubmit``` isn't really necessary. You can just use ```formIsValid``` instead.
The form can't be valid if it isn't filled in.