Just hit this exact problem when trying to install core snap:
linode:arch-2017.07.01 .../tests/unit/go# snap install --edge core
error: cannot perform the following tasks:
- Mount snap "core" (3542) ([start var-lib-snapd-snap-core-3542.mount] failed with exit status 1: Job for var-lib-snapd-snap-core-3542.mount failed.
See "systemctl status var-lib-snapd-snap-core-3542.mount" and "journalctl -xe" for details.
)
linode:arch-2017.07.01 .../tests/unit/go# systemctl status var-lib-snapd-snap-core-3542.mount
● var-lib-snapd-snap-core-3542.mount - Mount unit for core
Loaded: loaded (/etc/systemd/system/var-lib-snapd-snap-core-3542.mount; enabled; vendor preset: disabled)
Active: failed (Result: exit-code) since Thu 2017-11-23 13:15:37 UTC; 13s ago
Where: /var/lib/snapd/snap/core/3542
What: /var/lib/snapd/snaps/core_3542.snap
Process: 22034 ExecMount=/usr/bin/mount /var/lib/snapd/snaps/core_3542.snap /var/lib/snapd/snap/core/3542 -t squashfs -o nodev,ro (code=exited, status=1/FAILURE)
Nov 23 13:15:37 localhost systemd[1]: Mounting Mount unit for core...
Nov 23 13:15:37 localhost systemd[1]: var-lib-snapd-snap-core-3542.mount: Mount process exited, code=exited status=1
Nov 23 13:15:37 localhost systemd[1]: var-lib-snapd-snap-core-3542.mount: Failed with result 'exit-code'.
Nov 23 13:15:37 localhost systemd[1]: Failed to mount Mount unit for core.
linode:arch-2017.07.01 .../tests/unit/go# mount /var/lib/snapd/snaps/core_3542.snap /var/lib/snapd/snap/core/3542 -t squashfs -o nodev,ro
mount: /usr/lib/libmount.so.1: version `MOUNT_2.30' not found (required by mount)
Had to run pacman -Syu to install the updates and retry the operation.