Fix hang on setting invalid VFAT labels

Fix hang on setting invalid VFAT labels

VFAT does not support several characters in file names and labels, in
particular: " * / : < > ? \ |
(http://en.wikipedia.org/wiki/File_Allocation_Table)

When using one of those, mlabel interactively asks what to do, and thus causes
a hang. Now check for invalid characters and return a proper error instead.

Also update the test case to verify that we get an Error on invalid labels, and
that valid labels still work.