BIER CLI show commands; no crash on non-existant objects (VPP-1303)

DBGvpp# sh bier disp entry
DBGvpp# sh bier disp entry 0
No such BIER disp entry: 0
DBGvpp# sh bier disp table
DBGvpp# sh bier disp table 0
No such BIER disp table: 0
DBGvpp# sh bier disp table 11
No such BIER disp table: 11
DBGvpp#

DBGvpp# sh bier bift
no BIFT entries
DBGvpp# sh bier bift set 0
no BIFT entries
DBGvpp# sh bier bift set 0 sd 0 bsl 0
no BIFT entries
DBGvpp#

DBGvpp# sh bier fib
No BIER tables
DBGvpp# sh bier fib 0
DBGvpp# sh bier fib 0 4

DBGvpp# sh bier fmask
DBGvpp# sh bier fmask 2
No BIER f-mask 2

DBGvpp# sh bier imp
DBGvpp# sh bier imp 0
No such BIER imposition: 0

Change-Id: Ibadac3441dd8a6d1b96bd9ee4358e28498875b95
Signed-off-by: Neale Ranns <nranns@cisco.com>