Unsortable Columns Raise on FunctionClauseError

[jeremyjh]

This should be a more explicit exception type:

** (exit) an exception was raised:                                                                                                                                                              
    ** (FunctionClauseError) no function clause matching in anonymous fn/2 in PhoenixDatatables.Query.do_sorts/2                                                                                
        (phoenix_datatables) lib/phoenix_datatables/query.ex:51: anonymous fn({:error, "stats.date_all_stock_exhausted is not a sortable column."},