tests: remove UnexpectedInput from lttng_live_server.py

I don't think this exception is useful, versus just raising a
RuntimeError.

The current code catches exceptions at the top-level, logs it (using
logging.error) and prints it to stderr.  I don't think it's useful to
log using logging.error, since it also goes to stderr.  And I don't
think it's useful to manually print the exception.  Instead, let them
escape and let Python print them using its regular uncaught exception
mechanism, which ends up printing them on stderr too.

Change-Id: Ice797d4b672dc5eba7312c4cabb2740806c37a2a
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10894
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>