Restarting MongoDB automatically if it crashes

I found the answer myself: the simplest way to achieve that is to add the following two lines at the end of the upstart script installed by MongoDB (/etc/init/mongodb.conf):

respawn
respawn limit 10 90

This will try to restart the process if it terminates, and stop if it crashes more than 10 times in 90 seconds.

Building on the accepted answer (and since I do not have enough rep to comment on it)

Respawn is NOT a mongo configuration parameter, it is for the upstart service configuration file.

respawn
respawn limit 10 90

Digital Ocean has a nice tutorial for using upstart and systemd (newer). https://www.digitalocean.com/community/tutorials/how-to-configure-a-linux-service-to-start-automatically-after-a-crash-or-reboot-part-1-practical-examples