Getting rid of inexistent .webm file error message

Doc/Makefile

@@ -33,7 +33,7 @@ SAXONCMD=java -cp "/usr/share/java/Saxon-HE.jar:/usr/share/java/xercesImpl.jar"
 	avconv -i $< -acodec libx264  -c:a libvorbis -r 10 $<.mp4 || rm -f $<.mp4
 	avconv -i $< -vframes 1 $<.png || rm -f $<.png
 
-webmSet = $(shell /bin/ls target/*/Ref/Video/*.webm)
+webmSet = $(shell /bin/ls target/*/Ref/Video/*.webm 2>/dev/null)
 mp4Set  = $(foreach webmFile, $(webmSet), $(webmFile).mp4 )
 
 videoformats:$(mp4Set)