Otherwise, you just see an anonymous failure in the logs, like:

(00.244773) mnt: Dumping mountpoints
(00.244781) mnt:        253: 42:/ @ ./dev/.lxc
(00.244793) mnt: Path `/dev/.lxc' resolved to `./dev/.lxc' mountpoint
(00.249039) mnt:        133: 41:/ @ ./dev/hugepages
(00.249052) mnt:        116: 40:/ @ ./run/lock
(00.249064) mnt: Path `/run/lock' resolved to `./run/lock' mountpoint
(00.251948) mnt:        115: 3f:/ @ ./run
(00.251970) mnt: Something is mounted on top of ./run
(00.264803) mnt: Path `/run' resolved to `./run' mountpoint
tar: ./uuidd/request: socket ignored
tar: ./dbus/system_bus_socket: socket ignored
tar: ./acpid.socket: socket ignored
tar: ./systemd/journal/syslog: socket ignored
tar: ./systemd/journal/dev-log: socket ignored
tar: ./systemd/journal/socket: socket ignored
tar: ./systemd/journal/stdout: socket ignored
tar: ./systemd/private: socket ignored
tar: ./systemd/notify: socket ignored
(00.368950) mnt:        113: 3e:/ @ ./dev/shm
(00.368978) mnt: Path `/dev/shm' resolved to `./dev/shm' mountpoint
(00.371551) mnt:        112: 3d:/ @ ./dev/pts
(00.371566) mnt:        249: 28:/ @ ./var/lib/lxcfs
(00.371749) Unlock network
(00.371762) Running network-unlock scripts
(00.371766) Unfreezing tasks into 1
(00.371770)     Unseizing 13683 into 1
(00.371784)     Unseizing 14839 into 1
(00.371794)     Unseizing 15678 into 1
(00.371800)     Unseizing 16126 into 1
(00.371805)     Unseizing 16177 into 1
(00.371814)     Unseizing 16301 into 1
(00.371843)     Unseizing 16323 into 1
(00.371854)     Unseizing 16414 into 1
(00.371900)     Unseizing 16595 into 1
(00.371911)     Unseizing 16695 into 1
(00.371942)     Unseizing 16744 into 1
(00.371964)     Unseizing 16904 into 1
(00.372103)     Unseizing 16944 into 1
(00.372140)     Unseizing 17558 into 1
(00.372196) Error (cr-dump.c:1631): Dumping FAILED.

...which is hard to debug.
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>