dirvish does not delete lock file on some failures

[Mitch Claborn]

dirivsh does not delete the dirvish/lock_file under some error conditions, specifically when the client server being backed up is unreachable and there is a pre-client command specified in the default.conf file. Below are the details from one such example. This happens frequently since upgrading to Ubuntu 16.04.

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Contents of default.conf
client: user@hostname
tree: /home/debbie
pre-server: cp /backup2/dirvish/debbieb/dirvish/default.conf /etc/dirvish/confs/debbieb.conf
pre-client: /usr/bin/sudo /some/script
post-client: /some/other/script

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June 29 - the client server is down. Result of the dirvish run:

19:32:29 /usr/sbin/dirvish --vault debbieb
ssh: connect to host debbieb.mmhatch.com port 22: No route to host
debbieb:default pre-client failed (65280)

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June 30
debbieb:default:2018-06-30-19-38: /backup2/dirvish/debbieb/dirvish/lock_file already exists

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ls -alh /backup2/dirvish/debbieb/dirvish/lock_file

-rw-r--r-- 1 root root 4 Jun 29 19:32 /backup2/dirvish/debbieb/dirvish/lock_file

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: dirvish 1.2.1-1.3
ProcVersionSignature: Ubuntu 4.4.0-128.154-generic 4.4.131
Uname: Linux 4.4.0-128-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.18
Architecture: amd64
Date: Mon Jul 2 09:22:59 2018
InstallationDate: Installed on 2018-05-22 (41 days ago)
InstallationMedia: Ubuntu-Server 16.04.4 LTS "Xenial Xerus" - Release amd64 (20180228)
PackageArchitecture: all
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: dirvish
UpgradeStatus: No upgrade log present (probably fresh install)
modified.conffile..etc.cron.d.dirvish: [deleted]