Catch assertions in flow_test and print their values.

[OfirMos]

Currently when asset fails in a flow_test in RTS the message isn't printed, it can be catched and printed:
nose package implements this, see in one of the replies here:
https://stackoverflow.com/questions/1308607/python-assert-improved-introspection-of-failure

Example:
test_ts_range:test_agg_avg
[ERROR]
Unhandled exception:
Traceback (most recent call last):
File "/usr/local/lib/python3.6/dist-packages/RLTest/main.py", line 543, in _runTest
fn()
File "/home/RedisTimeSeries/tests/flow/test_ts_range.py", line 251, in test_agg_avg
assert actual_result == [[0, b'1.7976931348623157E308']]
AssertionError