bt2: check that port is connected when creating message iterator

Creating a message iterator for a non-connected input port hits a
precondition assertion.  Add some checks in the Python bindings to raise
a more friendly ValueError in that case.

Change-Id: I323e49c7408f5df8e1702eb993933ffc251b6c63
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3396
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>