rcutorture: Flag errors and warnings with color coding

The output of the rcutorture scripts often requires interpretation, so
this commit simplifies this interpretation by tagging messages as
BUGs (colored red) or WARNINGs (colored yellow).

Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Greg KH <gregkh@linuxfoundation.org>