bt2: make `bt2._OverflowError` inherit `bt2._Error`

bt2: make `bt2._OverflowError` inherit `bt2._Error`

`bt2._OverflowError` represents the `__BT_FUNC_STATUS_OVERFLOW_ERROR`
status code, which is an error. The library appends an error cause when
returning this status, so `bt2._OverflowError` inherits `bt2._Error`.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Iaf59bd604f595e8b57d46d7edec57b462e45416e
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1776
Tested-by: jenkins <jenkins@lttng.org>