Nothing. Why should anything happen? It should put an error in the log and continue trying to boot. If it reaches a point where it can go no further due to the missing volume. *then* it should drop back to the recovery console if necessary. If the volume is critical to the boot process it should be marked as such, in which case it would still stop at this point if missing.Not really.
Let's suppose a mount fails. What should happen next ?
Obviously, if / (or /bin, /usr, /home) doesn't mount, the answer is quite easy, most of the time.
However, let us suppose /thing/weird/dir doesn't mount, what should happen ? Maybe something in /etc requires something there, so the boot should fail, but how could systemd know this ?