Some validation fixes

[Sadique-The-Alchemist]

This PR is composed of two things 1. Small fix in mapping field validation: Contex.Mapping ensures the keys received on opts.mapping are valid by referring with a module attribute @expected_fields, but the MapSet.compare is not giving proper output, so that had to update an orientation of params 2. Arithmatic error on generating pie chart: The pie chart is generated by the value divided by the total of the values, if the total is zero it raises an arithmetic error doesn't contain enough details, so added an improved error message