task: don't hide exceptions in Task

task: don't hide exceptions in Task

If the Task raised an exception we currently hide it
and replace it with RuntimeError.  This is bad.

If an exception occured, then we don't have a result so
re-raise the original exception.

Or we could log the traceback and raise RuntimeError, but
that doesn't seem to be a good idea.

Sample traceback after re-raising original.  Without this patch the ValueError is only written
to _write_error_data

2018-03-25 22:57:56,511 yardstick.benchmark.contexts.node node.py:85 DEBUG BareMetals: []
2018-03-25 22:57:56,511 yardstick.benchmark.contexts.node node.py:89 DEBUG Env: {}
2018-03-25 22:57:56,511 yardstick.cmd.commands.task task.py:57 INFO Task FAILED
Traceback (most recent call last):
  File "/home/rbbratta/yardstick-upstream/yardstick/yardstick/cmd/commands/task.py", line 54, in do_start
    result = Task().start(param, **kwargs)
  File "/home/rbbratta/yardstick-upstream/yardstick/yardstick/benchmark/core/task.py", line 103, in start
    task_args_fnames)
  File "/home/rbbratta/yardstick-upstream/yardstick/yardstick/benchmark/core/task.py", line 321, in _parse_tasks
    task_args_fnames[i]
  File "/home/rbbratta/yardstick-upstream/yardstick/yardstick/benchmark/core/task.py", line 558, in parse_task
    context.init(cfg_attrs)
  File "/home/rbbratta/yardstick-upstream/yardstick/yardstick/benchmark/contexts/heat.py", line 131, in init
    server = Server(name, self, server_attrs)
  File "/home/rbbratta/yardstick-upstream/yardstick/yardstick/benchmark/contexts/model.py", line 210, in __init__
    (name, p))
ValueError: server 'trafficgen_1', placement 'pgrp2' is invalid
2018-03-25 22:57:56,512 yardstick.cmd.commands.task task.py:62 INFO Task FAILED
2018-03-25 22:57:56,662 yardstick.benchmark.runners.base base.py:124 DEBUG Terminating all runners
NoneType

JIRA: YARDSTICK-1102

Change-Id: I7e6fa41fc1d36f6d438a1602ab60cb41ffbee1e9
Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>