Misleading error message on LVM backup

[Zahari Zahariev]

When you do not pass the correct or if you ommit '--lvm-snapsize' option and you may have a smaller LVM volume than the default 5GB you will get the following error:

test_lvm_incremental_level30 (backup_scenario.BackupScenarioFS) ... ERROR
test_lvm_level0 (backup_scenario.BackupScenarioFS) ... ok
test_no_lvm_level0 (backup_scenario.BackupScenarioFS) ... ok
test_utils_methods (backup_scenario.BackupScenarioFS) ... ok

======================================================================
ERROR: test_lvm_incremental_level30 (backup_scenario.BackupScenarioFS)
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Traceback (most recent call last):
  File "/home/vagrant/freezer/tests/scenario/backup_scenario.py", line 306, in test_lvm_incremental_level30
    main.freezer_main(backup_args)
  File "/home/vagrant/freezer/freezer/main.py", line 30, in freezer_main
    freezer_job.execute()
  File "/home/vagrant/freezer/freezer/job.py", line 47, in wrapper
    retval = func(self)
  File "/home/vagrant/freezer/freezer/job.py", line 111, in execute
    self.conf, self.start_time.timestamp, manifest_meta_dict)
  File "/home/vagrant/freezer/freezer/backup.py", line 125, in backup_mode_fs
    lvm_snap(backup_opt_dict)
  File "/home/vagrant/freezer/freezer/lvm.py", line 195, in lvm_snap
    raise Exception('lvm snapshot mounting error: {0}'.format(mount_err))
Exception: lvm snapshot mounting error: mount: special device /dev/freezer-test1-volgroup/freezer_backup_snap does not exist

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Ran 4 tests in 2.585s
FAILED (errors=1)

Which does not communicate the real LVM error.