vp8enc: Return FLOW_ERROR when an error accures
vp8enc: Return FLOW_ERROR when an error accures

FALSE would mean FLOW_OK

https://bugzilla.gnome.org/show_bug.cgi?id=760666

hizukiayaka/gst-plugins-good,hizukiayaka/gst-plugins-good,Kurento/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good,stfl/gst-plugins-good,pexip/gst-plugins-good,Kurento/gst-plugins-good,GrokImageCompression/gst-plugins-good,stfl/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,stfl/gst-plugins-good,StreamUtils/gst-plugins-good,GrokImageCompression/gst-plugins-good,GrokImageCompression/gst-plugins-good,Kurento/gst-plugins-good,Kurento/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,StreamUtils/gst-plugins-good,GrokImageCompression/gst-plugins-good,StreamUtils/gst-plugins-good,Kurento/gst-plugins-good,GStreamer/gst-plugins-good,hizukiayaka/gst-plugins-good,stfl/gst-plugins-good,pexip/gst-plugins-good,hizukiayaka/gst-plugins-good