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>

subsys/bluetooth/audio/micp_mic_ctlr.c

@@ -416,6 +416,10 @@ static uint8_t micp_discover_func(struct bt_conn *conn,
 			} else {
 				LOG_DBG("Could not subscribe to handle 0x%04X: %d", attr->handle,
 					err);
+
+				micp_mic_ctlr_discover_complete(mic_ctlr, err);
+
+				return BT_GATT_ITER_STOP;
 			}
 		}
 	}