Encoder (codec id 145) not found for output stream #0.0

If ever you get the error "Encoder (codec id 145) not found for output stream #0.0" while convert to webm format, you need to install libvpx and add --enable-libvpx to the FFmpeg ./configure line.
just do the following:-

Install libvpx
git clone git://review.webmproject.org/libvpx
cd libvpx
./configure
make
sudo checkinstall --pkgname=libvpx --pkgversion="1:$(date +%Y%m%d%H%M)-git" --backup=no \
--deldoc=yes --fstrans=no --default

Install FFmpeg
git clone git://git.videolan.org/ffmpeg
cd ffmpeg
./configure --enable-gpl --enable-libfaac --enable-libmp3lame --enable-libopencore-amrnb \
--enable-libopencore-amrwb --enable-libtheora --enable-libvorbis --enable-libx264 \
--enable-libxvid --enable-nonfree --enable-postproc --enable-version3 --enable-x11grab --enable-libvpx
make
sudo checkinstall --pkgname=ffmpeg --pkgversion="5:$(date +%Y%m%d%H%M)-git" --backup=no \
--deldoc=yes --fstrans=no --default
hash x264 ffmpeg ffplay ffprobe

It will work. Enjoy :-)