fate: rename -error option to -error_rate.

This fixes fate when FF_API_ERROR_RATE=0.

tests/fate/vcodec.mak

@@ -211,7 +211,7 @@ fate-vsynth%-mpeg4-adv:          ENCOPTS = -qscale 9 -flags +mv4+aic       \
 
 fate-vsynth%-mpeg4-error:        ENCOPTS = -qscale 7 -flags +mv4+aic    \
                                            -data_partitioning 1 -mbd rd \
-                                           -ps 250 -error 10
+                                           -ps 250 -error_rate 10
 
 fate-vsynth%-mpeg4-nr:           ENCOPTS = -qscale 8 -flags +mv4 -mbd rd -nr 200