Change behavior of non-strict sequential units

Adjust the implementation of sequential units to the documentation,
according to which the execution of a non-strict sequential unit has to
continue even when one of the sub-unit fails. See:
https://dev.eclipse.org/mhonarc/lists/henshin-user/msg00141.html

Change-Id: I6f2698e6e9051bbdf6839413903a55226b45197a
1 file changed