Idea Summary
The Validation Error Message currently is a hard coded message, like

#LABEL# must have some value.

The idea is to add Text Message as option. Maybe in the form of

('REQUIRED_ITEM', '#LABEL#')

Or as an Expression

apex_lang.message('REQUIRED_ITEM', '#LABEL#')

Use Case
To avoid writing simple validations (like built-in Item is NOT NULL) to “Function Body (returning Error Text)”.

To avoid modifying all messages on all  pages if a widely used message needs to be changed, e.g.

#LABEL# must have some value => Please enter a value for #LABEL#

Text messages can be easily translated and don't have to go through the whole Seed and Publish cycle.