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).
How else could this be done without having the validation be in two different places?

Perhaps formIsValid should directly call validateForm()?

```
$scope.formIsValid = function() {
  try {
    validateForm();
  } catch (e) {
    return false;
  }
};
return true
```