Default to "r" mode if IAdmin.canUpdate fails (Fix #10657)

Any call to `RawFileStore.setFileId` with omero.group set to -1 was failing
with an InternalException because IAdmin.canUpdate was throwing. With this
change, RawFileBean assumes that if canUpdate fails that the mode should be
"r", meaning no changes are permissible to the file. Callers will need to
explicitly change the group id for "rw" behavior.

The major concern with this solution is that the action is silent: even the
owner of the file if calling with "-1" will not be able to make changes.