apng2webp: check if commands are available

.config/zsh/functions/apng2webp

@@ -17,6 +17,11 @@ fi
 local input=$(realpath ${1})
 local output=$(realpath ${2})
 
+autoload die
+for cmd in apngdis cwebp webpmux; do
+    type ${cmd} >& - || die "ERROR: ${cmd} not found"
+done
+
 local tmpdir=$(mktemp --directory --suffix='.apng2webp')
 pushd ${tmpdir}