Delete chgrp mif bugs may20th

Fix Bug #10889 and other minor Activities dialog issues with chgrp and delete of MIFs. See screenshots BEFORE:

![screen shot 2013-05-20 at 10 43 10](https://f.cloud.github.com/assets/900055/531264/f52e8986-c13a-11e2-9d86-2369f4781d44.png)

and AFTER:

![screen shot 2013-05-20 at 11 45 47](https://f.cloud.github.com/assets/900055/531266/fec2ac7a-c13a-11e2-9768-aebf6774d02c.png)


To test:
 - Delete part of a fileset & then try to Chgrp part of a fileset. See that the failure messages are more similar (don't mention number or size of files in fileset).
 - Also see that that "fileset" is always lowercase and is called "multi-image fileset" the first time that the term 'fileset' is used.
 - Delete all the images in a fileset. Should see no error messages.