perform more extensive checks on dryrun

So i was in the process of encrypting my entire system and recreating all my btrfs volumes/subvolumes in the process i made a mistake and created my snapshot dir as @Backup instead of @Backups like was set up originally and defined in btrbk.conf. When i ran dryrun it did not show me any errors:

```
btrbk -c /etc/btrbk/btrbk.conf dryrun
--------------------------------------------------------------------------------
Backup Summary (btrbk command line client, version 0.19.2)

Date: Thu Jun 11 08:43:35 2015
Config: /etc/btrbk/btrbk.conf

Legend:
=== up-to-date subvolume (source snapshot)
+++ created subvolume (source snapshot)
--- deleted subvolume
*** received subvolume (non-incremental)
>>> received subvolume (incremental)
--------------------------------------------------------------------------------
/mnt/btrfsroot/@Home
+++ /mnt/btrfsroot/@Backups/@Home.20150611
*** /media/Storage3/Backup/@HomeBackup/@Home.20150611

/mnt/btrfsroot/@Storage
+++ /mnt/btrfsroot/@Backups/@Storage.20150611
*** /media/Storage3/Backup/@StorageBackup/@Storage.20150611

NOTE: Dryrun was active, none of the operations above were actually executed!
```

but then when i ran run it failed(as it should since the @Backups subvol is not present):

```
btrbk -v -c /etc/btrbk/btrbk.conf run
btrbk command line client, version 0.19.2 (Thu Jun 11 08:44:04 2015)
Using configuration: /etc/btrbk/btrbk.conf
Creating subvolume snapshot for: /mnt/btrfsroot/@Home
>>> /mnt/btrfsroot/@Backups/@Home.20150611
ERROR: can't access '/mnt/btrfsroot/@Backups'
ERROR: Failed to create btrfs subvolume snapshot: /mnt/btrfsroot/@Home -> /mnt/btrfsroot/@Backups/@Home.20150611
WARNING: Skipping subvolume section: Failed to create snapshot: /mnt/btrfsroot/@Home -> /mnt/btrfsroot/@Backups/@Home.20150611
Creating subvolume snapshot for: /mnt/btrfsroot/@Storage
>>> /mnt/btrfsroot/@Backups/@Storage.20150611
ERROR: can't access '/mnt/btrfsroot/@Backups'
ERROR: Failed to create btrfs subvolume snapshot: /mnt/btrfsroot/@Storage -> /mnt/btrfsroot/@Backups/@Storage.20150611
WARNING: Skipping subvolume section: Failed to create snapshot: /mnt/btrfsroot/@Storage -> /mnt/btrfsroot/@Backups/@Storage.20150611
Completed within: 0s (Thu Jun 11 08:44:04 2015)
--------------------------------------------------------------------------------
Backup Summary (btrbk command line client, version 0.19.2)

Date: Thu Jun 11 08:44:04 2015
Config: /etc/btrbk/btrbk.conf

Legend:
=== up-to-date subvolume (source snapshot)
+++ created subvolume (source snapshot)
--- deleted subvolume
*** received subvolume (non-incremental)
>>> received subvolume (incremental)
--------------------------------------------------------------------------------
/mnt/btrfsroot/@Home
!!! Aborted: Failed to create snapshot: /mnt/btrfsroot/@Home -> /mnt/btrfsroot/@Backups/@Home.20150611

/mnt/btrfsroot/@Storage
!!! Aborted: Failed to create snapshot: /mnt/btrfsroot/@Storage -> /mnt/btrfsroot/@Backups/@Storage.20150611

NOTE: Some errors occurred, which may result in missing backups!
```

So it seems that some part of dryrun is not testing for the presence of the snapshot dir as it should.

```
btrfs subvol list /mnt/btrfsroot
ID 259 gen 9 top level 5 path @Backup
ID 646 gen 1303 top level 5 path @Home
ID 901 gen 1307 top level 5 path @Storage
```

Reproduce the reported case with `btrbk -c /etc/btrbk/btrbk.conf dryrun` and compare it with `btrbk -v -c /etc/btrbk/btrbk.conf run` when `@Backup` exists but `@Backups` does not. Trace the dry-run handling for the snapshot destination; done when dryrun reports the missing directory instead of presenting operations that run will fail.

Written by the indexing model from the issue text.