[gen] Remove log.Fatal and return error in generated code

[pingginp]

In code generated from schema, in some switch statement log.Fatal is used, should just return error

func Map{{$StructType}}(iter *gocql.Iter, callback func(t {{$StructType}}) (bool, error)) error {
        columns := iter.Columns()
        row := make([]interface{}, len(columns))

        for {
            t := {{$StructType}}{}

            for i := 0; i < len(columns); i++ {
                switch columns[i].Name {
                {{range $_, $col := $cf.Columns}}
                    case "{{$col.Name}}": row[i] = &t.{{snakeToCamel $col.Name}}
                {{end}}
                default:
                    log.Fatal("unhandled column: ", columns[i].Name)
                }
            }
    }