Signing form: add a red asterisk on mandatory fields and add feedback messages when mandatory fields are not filled in

On the Signing form in the Signature tab of documents, both the comment and the password are mandatory,but there is no indication to the user. We should add a red asterisk to indicate that they are mandatory.

Beside that, if the user clicks on the the Sign button without filling one of these 2 fields, he has no feedback message displayed to tell him chat is wrong.