lib: rename INVALID_OBJECT status to UNKNOWN_OBJECT

lib: rename INVALID_OBJECT status to UNKNOWN_OBJECT

Rename INVALID_OBJECT to UNKNOWN_OBJECT, because it represents better
the situation: the passed object is not necessarily invalid (any string
is a valid object), it's just that the component class doesn't know that
object.

Change its numerical value to something positive, because it should not
be considered as an error (which negative numerical values represent).
A function returning UNKNOWN_OBJECT should not append an error cause.

Also, in the Python bindings, change the default implementation of
_query to raise bt2.UnknownObject instead of NotImplementedError.
NotImplementedError would result in an ERROR status code, whereas
bt2.UnknownObject becomes UNKNOWN_OBJECT.  This matches the behavior of
a component class implemented in C, where if it doesn't provide a query
method, it will automatically return UNKNOWN_OBJECT.

Change-Id: Ica1418272b5bf2bbe8e96d639c16f56191151d79
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1760
CI-Build: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>