Nitin,
Statistics are available at exception group level by standard programs. You can use reports RMMDAS10 (you need to maintain proper selections in configuration) or IP_MDAS10_46C programs to get group wise statistics.
However, if you need exception message wise statistics you can get a custom report developed using function BAPI_MATERIAL_MRP_LIST or MD_MRP_LIST_API (not released for customers and you should use the BAPI version).
Best Regards.
Rajen
