AddMembers.apply: Prevent NPE when account doesn't exist

When trying to add an account to a group, if the account doesn't
exist yet and cannot be created, AddMembers.apply() would throw
an NPE. Fix it to throw UnprocessableEntityException instead,
which is handled by RestApiServlet.

Signed-off-by: Jay Soffian <jaysoffian@gmail.com>
Change-Id: I724a7b8a95a8df70506ae591575c74153278adc4
Merge "AddMembers.apply: Prevent NPE when account doesn't exist" into stable-2.9