iwlagn: Improve aggregation failure error messages

82ca9341763107615a15da6e59b9535d49eb91c3 added scary looking
but harmless error messages.  Make them clearer and make the
actual failure message show up with the same severity as the
harmless one.

Signed-off-by: Andy Lutomirski <luto@mit.edu>
Signed-off-by: John W. Linville <linville@tuxdriver.com>