target: fixup error message in target_tg_pt_gp_tg_pt_gp_id_store()

When setting up an ALUA target port group with an invalid ID the
error message

kstrtoul() returned -22 for tg_pt_gp_id

is displayed, which is not really helpful.
Convert it to something sane.
And while we're at it, join the messages onto a single line.

Signed-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Bart van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>