handle exception when trying to submit tests that don't exist

[anderbubble]

Traceback (most recent call last):
  File "/home/joan5896/bin/bench", line 8, in <module>
    load_entry_point('bench==0.0.0', 'console_scripts', 'bench')()
  File "/home/joan5896/curc-bench/src/bench/driver.py", line 187, in driver
    account = args.account,
  File "/home/joan5896/curc-bench/src/bench/submit.py", line 32, in execute
    index = submit(os.path.join(directory, 'alltoall-rack', 'tests'), index, pause, **kwargs)
  File "/home/joan5896/curc-bench/src/bench/submit.py", line 51, in submit
    for test_basename in os.listdir(prefix):
OSError: [Errno 2] No such file or directory: './2015-05-07-1/alltoall-rack/tests'