improve ecryptfs-mount-private output at error

[Karl-Philipp Richter]

When ecryptfs-mount-private fails due to a missing file or diretory it should catch the exception/anticipate this state and provide useful error messages. It does not in the following example:
Steps to reproduce:
<code>
# Open system recovery and open root console
remount -o remount,rw /
init 3
su <user>
ecryptfs-mount-private
# enter login password
</code>
This will print
<output>
inserted auth tok with sig [...] into the user session keyring
open: No such file or directory
Error locking counter
</output>
user doesn't have any idea what went wrong, but output could be helpful, e.g. tell WHICH file or directory is missing.

version information depend on:
https://bugs.launchpad.net/ecryptfs/+bug/1297042

[Sam Pointer (sam.pointer)]

Should anybody trying to be mount an encrypted volume in 2017 from a recovery root prompt, the following steps work on Ubuntu 14.

The primary missing file is the symlink of `/dev/shm` to `/run/shm` and the permissions on the target when booted in this state.

export user="me"
chgrp ${user} /run/shm
chmod g+w /run/shm
cd /dev
ln -s /run/shm/ shm
mount -o remount,rw $(mount | grep "on / type" | awk '{print $1}') /
su - ${user}
/usr/bin/ecryptfs-mount-private

[Sam Pointer (sam.pointer)]

This specific problem seems to have been introduced in Ubuntu in 2011 with the move to the /run filesystem.[1] Unless /etc/init/mounted-dev.conf is run the symlink of /dev/shm, created in that script, will not be created.

The reason /dev/shm is expected is because `ecryptfs-mount-private` executes mount.ecryptfs_private, which is built to expect /dev/shm to exist.[2]

[1]: http://metadata.ftp-master.debian.org/changelogs/main/m/mountall/oldstable_changelog
[2]: http://bazaar.launchpad.net/~ecryptfs/ecryptfs/trunk/view/head:/src/utils/mount.ecryptfs_private.c#L55

[Sam Pointer (sam.pointer)]

The underlying reason for this error is rooted in the move to the /run filesystem in 2011. Unless /etc/init/mounted-dev.conf is run a symlink from /dev/shm to /run/shm is not created.[1]

/usr/bin/ecryptfs-mount-private itself executes /sbin/mount.ecryptfs_private, which in turn is built to exepect /dev/shm to exist.[2]

[1]: http://metadata.ftp-master.debian.org/changelogs/main/m/mountall/oldstable_changelog
[2]: http://bazaar.launchpad.net/~ecryptfs/ecryptfs/trunk/view/head:/src/utils/mount.ecryptfs_private.c#L55