Fix error message.

Fix error message.

Replace

| root@grml ~ # grml2usb --fat16 /live/image /dev/sdb1 --bootoptions="ssh=secret" --grub
| Executing grml2usb version 0.13.0
| Are you sure you want to format the specified partition with fat16? y/N y
| Note: you can skip this question using the option --force
| Formating partition with fat16 filesystem
| mkfs.vfat 3.0.13 (30 Jun 2012)
| Using /live/image as install base
| Error: could not find grml-version file.
| Traceback (most recent call last):
|   File "/usr/sbin/grml2usb", line 1738, in <module>
|     main()
|   File "/usr/sbin/grml2usb", line 1715, in main
|     install(iso, device)
|   File "/usr/sbin/grml2usb", line 1444, in install
|     install_grml(iso_mountpoint, device)
|   File "/usr/sbin/grml2usb", line 1478, in install_grml
|     grml_flavours = identify_grml_flavour(mountpoint)
|   File "/usr/sbin/grml2usb", line 1112, in identify_grml_flavour
|     raise
| TypeError: exceptions must be old-style classes or derived from
| BaseException, not NoneType
| 1 root@grml ~ #

with

| root@grml ~ # grml2usb --fat16 /live/image /dev/sdb1 --bootoptions="ssh=secret" --grub
| Executing grml2usb version 0.13.0
| Are you sure you want to format the specified partition with fat16? y/N y
| Note: you can skip this question using the option --force
| Formating partition with fat16 filesystem
| mkfs.vfat 3.0.13 (30 Jun 2012)
| Using /live/image as install base
| Error: could not find grml-version file.
| Cleaning up before exiting...
| 1 root@grml ~ #