Bluetooth: Host: Define bt_gatt_err_to_str()

The function reuses the ATT implementation.
To make the function simpler to use, the function handles both positive
and negative values.

Unfortunately the APIs do not document if the API returns an
errno val or a GATT return value.

Signed-off-by: Rubin Gerritsen <rubin.gerritsen@nordicsemi.no>