Constant CORRUPT_GROUPS_HINT 

const CORRUPT_GROUPS_HINT: &str = "dplyr's validators reject such a frame too. Strip the \
     grouping with dplyr::ungroup(), then regroup it with dplyr::group_by().";

The fix that the corrupt-groups errors (DataFrameError::GroupsNotDataFrame, DataFrameError::MissingGroupRows, DataFrameError::BadGroupRows) append to their message.