Fixex and error message

This makes use of an errorbox instead of an infobox for an error
message.
Merge branch 'hotfix-errorbox' into develop
[158] Implement constraint 28/29

This checks whether an object of type 'Merger' has at least two incoming
relations of type 'Subsequent' and ammends the log output message to
that effect.
Merge branch 'feature-158' into develop