fix #12868: prevent error when deleting container with undeletable content

[mtbc]

Fixes error in workflow from http://trac.openmicroscopy.org/ome/ticket/12868. Now a group owner Alice may delete Bob's dataset that includes images that Bob tagged.

--no-rebase

[pwalczysko]

On today's trout merge:

• the case mentioned in the http://trac.openmicroscopy.org/ome/timeline?from=2015-05-06T15%3A13%3A14%2B01%3A00&precision=second is passing now both in r-a as well as r-w groups (all yours, just the annotations not yours)
• Also the Bob and Alice case from the header of the PR works fine, r-a and r-w groups, also with other annotations (like file anttachments, comments, ROIs) present

[joshmoore]

LGTY?

[pwalczysko]

All the other cases mentioned in the testing sheet which failed on delete are passing now.
This seems all okay here.

[pwalczysko]

All looks good.