gst-launch: Fix process return value on error
gst-launch: Fix process return value on error

In case of a run-time error message, the process return value was left
unset. This would lead to error not being caught at shell level.

https://bugzilla.gnome.org/show_bug.cgi?id=759019

centricular/gstreamer,jpxiong/gstreamer,jpxiong/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer,centricular/gstreamer,jpxiong/gstreamer,centricular/gstreamer,centricular/gstreamer