Include failing mount command in the error dialog (#742741)

On RHEL/CentOS 6 GParted fails to mount nilfs2 file system.  Include the
failing file system mount command in the error dialog so the user knows
what command failed.  The error dialog now looks like:

    (-) Could not mount /dev/sdc1 on /mnt/1

        # mount -v /dev/sdc1 "/mnt/1"
        mount: you must specify the filesystem type

                                           [  OK  ]

Also stop telling the dialog that the secondary text contains pango
markup as the command line and error message certainly isn't pango
markup text.

Bug 742741 - Nilfs2 file system is unusable on RHEL/CentOS 6