Improve error message when non-integer value is entered in Integer widget

1. create custom document
2. add integer to document schema
3. add integer widget to document
4. synchronize the project with the server
5. enter a non-integer value or even 123.45
6. the error displayed is not so clear :

   intWidgetLayout_edit_form:nxl_intWidgetLayout:nxw_intWidget: An error occurred when processing your submitted information
   

This could be improved to a more specific error message like 'abc' is not a number. Example: 99