graph: Fix: bt_graph_configure() does not propagate errors

graph: Fix: bt_graph_configure() does not propagate errors

Issue
=====
When configuring the graph, message iterator initialize functions may fail
and return an error status (e.g. _STATUS_NOMEM and _STATUS_ERROR). Those
error status are not propagated back to the caller of
`bt_graph_configure()` function. The caller that will consider the graph
to be configured properly and run it will it assertions down the road.

Solution
========
Return `BT_GRAPH_STATUS_ERROR` if any of the sinks returned a non-OK
status from its `graph_is_configured()` function.

Known drawbacks
===============
None.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>