Add missing error message when adding agregates on page provider

Steps to reproduce:

• Create a new Page Provider.
• Add a new Date Range aggregate but do not fill in the ranges, only the labels.
• Click Save.
  => The following error message appears:

  Error
  An error occurred!
  
  No Error Message

  => Clicking on the Show Details button provides no helpful information.

Expected: no error message should be shown. Studio should allow putting no range (could be a valid use case even if unlikely as it would mean "get me everything" which is usually not what you want when you set up filters...)