fix(gmerge): Fix KeyError handling when update.conf invalid

### Before:
```
core@localhost ~ $ /usr/bin/gmerge coreos-base/update_engine
Traceback (most recent call last):
  File "/usr/bin/gmerge", line 136, in <module>
    main()
  File "/usr/bin/gmerge", line 121, in main
    merger = GMerger(conf_data)
  File "/usr/bin/gmerge", line 31, in __init__
    sys.exit('Could not find /etc/coreos/update.conf value: ' + e)
TypeError: cannot concatenate 'str' and 'exceptions.KeyError' objects
```

### After: 
```
core@localhost ~ $ ./gmerge coreos-base/update_engine
Could not find /etc/coreos/update.conf value: DEVSERVER
```

Related: https://github.com/coreos/docs/pull/212