Fix problems with detection of USB drives on linux and exceptions handling

Not all linux distributions mount USB drives to /media. We should
rather iterate through all mountpoints, which can be found in
/proc/mounts, and test them.
Also, exceptions raised in GetAllFatxDrives() should be handled in
GUI so we can show an error dialog.
Merge pull request #58 from Kuci/master

Fix problems with detection of USB drives on linux and exceptions handling