Using g_error() crashes the application, producing a coredump e.g. when
the user closes the video window, which can be confusing (especially on
the very first tutorial).
Let's change this to log the error message without crashing, using
g_printerr(), like subsequent tutorials.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/6883>
85fa81c7c8