Improve Form element to fullfill the WCAG 2.0 level AA compliance

Description

According to the discussion at https://www.yammer.com/emvp/#/threads/show?threadId=1083938207

We need to improve form element behavior, such as input validation, so the user can fix any errors.

• If there is an error when submitting, set the input element's aria-invalid attribute = true.
• Add the input element's aria-describedby attribute to the validation error message.
• Added the alt attribute to some Img elements.
• Removed the placeholder attribute if it is empty.
• Fixed the tab order of the ImageChoiceElement so the validation works.
• If the submit action fails, the image is focused as it is at the first invalid element.