Verify that attribute table does not get corrupted when graph modification fails

[szhorvat]

This task is to verify that the attribute table does not get corrupted when graph modification fails. This issue is specifically for the attribute handler of python-igraph.

This relates to https://github.com/igraph/igraph/issues/1976 and https://github.com/igraph/igraph/issues/1999

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 14 days if no further activity occurs. Thank you for your contributions.

[ntamas]

Working on this, see 1d5a310 (more to come)

[ntamas]

Fixed ultimately in a30ca7c