ShellPkg/Ifconfig6: Update error message and add a new line

ShellPkg/Ifconfig6: Update error message and add a new line

If DAD failed, we print "It failed to set", print the IPv6
address and exit. We need a better error string
and also, after printing the IPv6 address, we need a new line
else the Shell> prompt will overwrite the IPv6 address. Fixed
these issues in this patch.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hegde Nagaraj P <nagaraj-p.hegde@hpe.com>
Reviewed-by: Sriram Subramanian <sriram-s@hpe.com>
Reviewed-by: Wu Jiaxin <jiaxin.wu@intel.com>