gif2webp: only write error messages to stderr
gif2webp: only write error messages to stderr

also output something if -o is not given, but the process succeeded

Change-Id: I466c377f0cbf8c9f45bb1839191b04dada9c5f0f

zofuthan/libwebp,jiehu5114/libwebp,Maria1099/webm.libwebp,comoyo/gonzo-libwebp,ya7lelkom/libwebp,PKRoma/libwebp,redengineer/libwebp,kalli123/webm.libwebp,Maria1099/webm.libwebp,comoyo/gonzo-libwebp,kleopatra999/webm.libwebp,kalli123/webm.libwebp,appunite/libwebp,keane83/BLE_IM,ttyangf/libwebp,rxl194/libwebp,yohunl/libwebp,ya7lelkom/libwebp,jacklicn/libwebp,gshORTON/webm.libwebp,Distrotech/libwebp,jacklicn/libwebp,reimaginemedia/webm.libwebp,Acidburn0zzz/webm.libwebp,imazen/libwebp,Aexyn/libwebp,reimaginemedia/webm.libwebp,Suvarna1488/webm.libwebp,comoyo/gonzo-libwebp,appunite/libwebp,kleopatra999/webm.libwebp,rxl194/libwebp,matanbs/webm.libwebp,zofuthan/libwebp,gshORTON/webm.libwebp,ya7lelkom/libwebp,zofuthan/libwebp,webmproject/libwebp,Tekserve/libwebp,redengineer/libwebp,iniwf/webm.libwebp,ericmckean/webm.libwebp,Maria1099/webm.libwebp,imazen/libwebp,kim42083/webm.libwebp,jacklicn/libwebp,zofuthan/libwebp,PKRoma/libwebp,zofuthan/libwebp,rxl194/libwebp,iniwf/webm.libwebp,ttyangf/libwebp,Aexyn/libwebp,imazen/libwebp,matanbs/webm.libwebp,ya7lelkom/libwebp,reimaginemedia/webm.libwebp,ericmckean/webm.libwebp,redengineer/libwebp,jacklicn/libwebp,Tekserve/libwebp,cysp/libwebp,matanbs/webm.libwebp,ericmckean/webm.libwebp,yohunl/libwebp,Aexyn/libwebp,Acidburn0zzz/webm.libwebp,imazen/libwebp,kleopatra999/webm.libwebp,rxl194/libwebp,iniwf/webm.libwebp,kleopatra999/webm.libwebp,ericmckean/webm.libwebp,appunite/libwebp,kalli123/webm.libwebp,kim42083/webm.libwebp,appunite/libwebp,altogother/webm.libwebp,kleopatra999/webm.libwebp,PKRoma/libwebp,imazen/libwebp,abwiz0086/webm.libwebp,comoyo/gonzo-libwebp,ttyangf/libwebp,gshORTON/webm.libwebp,abwiz0086/webm.libwebp,gshORTON/webm.libwebp,keane83/BLE_IM,abwiz0086/webm.libwebp,kim42083/webm.libwebp,jacklicn/libwebp,yohunl/libwebp,kalli123/webm.libwebp,comoyo/gonzo-libwebp,Tekserve/libwebp,Aexyn/libwebp,kim42083/webm.libwebp,Distrotech/libwebp,PKRoma/libwebp,redengineer/libwebp,webmproject/libwebp,matanbs/webm.libwebp,matanbs/webm.libwebp,iniwf/webm.libwebp,ttyangf/libwebp,Suvarna1488/webm.libwebp,cysp/libwebp,altogother/webm.libwebp,ttyangf/libwebp,rxl194/libwebp,reimaginemedia/webm.libwebp,PKRoma/libwebp,webmproject/libwebp,keane83/BLE_IM,comoyo/gonzo-libwebp,Acidburn0zzz/webm.libwebp,abwiz0086/webm.libwebp,webmproject/libwebp,kim42083/webm.libwebp,appunite/libwebp,Aexyn/libwebp,comoyo/gonzo-libwebp,Maria1099/webm.libwebp,Acidburn0zzz/webm.libwebp,keane83/BLE_IM,altogother/webm.libwebp,cysp/libwebp,kalli123/webm.libwebp,jiehu5114/libwebp,Distrotech/libwebp,Maria1099/webm.libwebp,yohunl/libwebp,keane83/BLE_IM,ya7lelkom/libwebp,webmproject/libwebp,iniwf/webm.libwebp,Tekserve/libwebp,gshORTON/webm.libwebp,abwiz0086/webm.libwebp,cysp/libwebp,altogother/webm.libwebp,Suvarna1488/webm.libwebp,Suvarna1488/webm.libwebp,jiehu5114/libwebp,appunite/libwebp,Acidburn0zzz/webm.libwebp,yohunl/libwebp,jiehu5114/libwebp,redengineer/libwebp,PKRoma/libwebp,altogother/webm.libwebp,ericmckean/webm.libwebp,Suvarna1488/webm.libwebp,jiehu5114/libwebp,reimaginemedia/webm.libwebp