vmtest: uncaught exception results in lage image files

On Sept 1, 2017 the vmtest run caught image files being archived:

3.9G output/ArtfulTestRaid6boot
3.9G output/ArtfulTestRaid6boot/disks
4.1G output/
+ '[' 1 -eq 2 ']'
+ exit 1
Build step 'Execute shell' marked build as failure
Archiving artifacts

Per Ryan's email these large files should never be stored:

CURTIN_VMTEST_KEEP_DATA_FAIL=logs,collect
CURTIN_VMTEST_KEEP_DATA_PASS=logs,collect

Ryan found that the cause appears to be an uncaught exception during curtin-log-print namely:

curtin.util.ProcessExecutionError: Unexpected error while running command.
Command: ['tools/curtin-log-print', '--dumpfiles', '/var/lib/jenkins/slaves/torkoal/workspace/curtin-vmtest-devel-amd64/output/ArtfulTestRaid6boot/logs', '/var/lib/jenkins/slaves/torkoal/workspace/curtin-vmtest-devel-amd64/output/ArtfulTestRaid6boot/logs/webhooks-events.json']
Exit code: 1
Reason: -
Stdout: ''
Stderr: Traceback (most recent call last):
          File "tools/curtin-log-print", line 197, in <module>
            main()
          File "tools/curtin-log-print", line 188, in main
            log_datafiles=opts.log_datafiles)
          File "tools/curtin-log-print", line 121, in generate_records
            timestamps[name].update({'finish': event['timestamp']})
        KeyError: 'cmd-install/stage-curthooks/builtin/cmd-curthooks/installing-missing-packages'