ssh kitten: Better error message when tar is missing

shell-integration/ssh/bootstrap.sh

@@ -92,6 +92,7 @@ read_base64_from_tty() {
 untar_and_read_env() {
     # extract the tar file atomically, in the sense that any file from the
     # tarfile is only put into place after it has been fully written to disk
+    command -v tar > /dev/null 2> /dev/null || die "tar is not availiable on this server. The ssh kitten requires tar."
     tdir=$(command mktemp -d "$HOME/.kitty-ssh-kitten-untar-XXXXXXXXXXXX")
     [ $? = 0 ] || die "Creating temp directory failed"
     # suppress STDERR for tar as tar prints various warnings if for instance, timestamps are in the future