modprobe: correct exitcode handling and error messages with respect to -q

modprobe: correct exitcode handling and error messages with respect to -q

function                                             old     new   delta
do_modprobe                                          319     339     +20
bb_delete_module                                      10      26     +16
moderror                                              62      71      +9
bb_init_module                                       112     119      +7
modprobe_main                                        488     494      +6
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(add/remove: 0/0 grow/shrink: 5/0 up/down: 58/0)               Total: 58 bytes

Signed-off-by: Gilles Espinasse <g.esp@free.fr>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>