Add back notifications in the case of errors

In the old runtime, we were using dlopen to send desktop notifications as a way to notify the user if somehting went wrong.

Since we no longer have the possibility to use dlopen, this was removed.

We should use the `notify-send` command if it is found on the `$PATH` instead.

Without this, the user gets no feedback whatsoever in the GUI in case somehting goes wrong.