If server is started without a JWT key, a message like the following is printed:
```
################################################################################
# ERROR:
# A value must be supplied for --auth_jwt_key. This value is used to create
# session tokens for users.
#
# Consider using the following randomly generated key:
# om3w95gMA2drT5xAdLd2Q5oE8fLw+Miz
################################################################################
```
Closes#1480.