Refactor constraint message to match core's required field message

Problem/Motivation

The core's validation message for required fields is:
'@name field is required.'
while the message provided by the module's constraint is:
'Field "%field_label" is required when publishing.'

Proposed resolution

I think it would be better to refactor the message to be more similar to the core's one:
'@field_label field is required when publishing.'