This resolves the case where /proc/sysrq-trigger doesn't exist by simply
ignoring any mount failure on ENOENT. With the current mount list, this
will always result in a safe environment (typically the read-only
underlay).

Closes #425

v2: Don't always show an error
Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>