harness: Abort when failed to construct the specified pipeline

harness: Abort when failed to construct the specified pipeline

gst_harness_new_parse() returns without any error even if it doesn't
find the specified element.  Then a succeeding call to
gst_harness_set_sink_caps_str() causes an error like this:

	Unexpected critical/warning: gst_pad_push_event: assertion 'GST_IS_PAD (pad)' failed

This is a bit cryptic and doesn't give users any clue what was going
on.

gst_harness_new_parse() calls gst_harness_add_parse() with a newly
created empty harness and the given pipeline description string, but
gst_harness_add_parse() does not have a way to propagate the error
back to the caller.  Since the function, gst_harness_add_parse(), is a
public API, it's not a good idea to change its signature.  This patch,
instead, makes the function to g_error() when it discovers any error.
With this change the same error prints:

	** (myelement-test:25345): ERROR **: Unable to create pipeline 'bin.( myelement )': no element "myelement"

The current implementation of gst_parse_launch_full() doesn't return
partially constructed pipeline when GST_PARSE_FLAG_FATAL_ERRORS is
specified, however, this patch also adds a check for it.

https://bugzilla.gnome.org/show_bug.cgi?id=781958