bt2: prepend `_` to names of exception classes the user cannot call

bt2: prepend `_` to names of exception classes the user cannot call

`bt2._OverflowError` and `bt2._IncompleteUserClass` are classes the
package's user cannot instantiate, so this patch adds the typical `_`
prefix to indicate that.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I42cb22311dac6d36dd7e57be86cc71d27ebab6a2
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1771
Tested-by: jenkins <jenkins@lttng.org>