Provide an option to customize text for the "required" indicator and validation error for fields.

In #5348 I've been working towards custom form validation errors for required Form API elements via a #required_error property. I believe that we should also allow people to customize the default "This field is required" text added to the "required" indicators (red asterisks) for fields added to content types and other entities:

We already have a "Required field" checkbox available in when specifying field settings:

What I would like us to have is: - a text field "tied" to that checkbox (hidden by default - shown only when the "Required field" checkbox is ticked), to allow people to customize the default text. - another text field "tied" to that checkbox, to allow customizing the validation error message for that required field.