crit: display help message when using python3

Running crit with python2 gives following minimal help message:

 $ crit/crit
 usage: crit [-h] {decode,encode,info,x,show} ...
 crit: error: too few arguments

Using a python3 only system crit shows the following error:

 $ crit/crit
 Traceback (most recent call last):
   File "crit/crit", line 6, in <module>
     cli.main()
   File "/home/criu/crit/pycriu/cli.py", line 334, in main
     opts["func"](opts)
 KeyError: 'func'

Using this patch the python3 output changes to:

 $ crit/crit
 usage: crit [-h] {decode,encode,info,x,show} ...
 crit: error: too few arguments

Suggested-by: Andrei Vagin <avagin@gmail.com>
Signed-off-by: Adrian Reber <areber@redhat.com>