Bluetooth: Audio: Add missing error checks for calls to bt_gatt_subscribe

Several places the LE Audio clients called bt_gatt_subscribe without
checking the return value, which could cause some issues in the worst
case, and in the best case, cause some unexpected behavior.

Some implementations had a bit more updating to handle the new
behavior.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>