Correct dialog title displaying libparted exceptions (#743825)

Encountered this dialog:

    Libparted Bug Found!

    (-) Invalid argument during seek for read on /dev/md1

                          [ Retry ] [ Cancel ] [ Ignore ]

After investigation found that GParted was asking libparted to read from
sector -1.  Libparted was actually reporting an error but GParted was
incorrectly displaying this as a bug.  This was because setting the
dialog title was falling through from the error case to the bug case.
Fix this.

Also provide unique titles for all libparted exception types so that
they can all be distinguished.

 *  List of exceptions from libparted: enum _PedExceptionType
    http://www.gnu.org/software/parted/api/group__PedException.html

Bug 743825 - gparted displaying dialog: libparted bug found!