nice alerts

### Done
* Removed alerts for empty form validation, now displaying error in red at bottom of form.
* Removed alerts for invalid access code, now displaying error in red at bottom of form.
* Refactored empty form validation.

### To do 
* Remains the alert in location validation. I could have done it in this pull request, but the location validation will soon use defers instead of callbacks so the message display will be called in the subscription module instead of in the location validation module. When this will be done, call displayError in subscription and remove alert in the location validation. I wrote 'location validation' 5 times in 4 lines.
For the same reason as previous comment, the function does more than just checking if the form is valid, it also displays an error if not. I wanted to avoid managing cases on the button click handler so I moved them in sub functions and renamed the functions to validate[...]() instead.