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 would avoid throwing and catching errors in favor of ```validateForm()``` returning a ```boolean``` and setting the error message on a scope variable. Then in the template you display the error message if it is defined.

Currency the validation runs twice. Once when ```fromIsValid()``` is called and again when the template evaluates ```formErrorMessage```. You should call validateForm when the input changes, and rely on the results of that validation (without calling it again) for all of the other function.