Prevents 'OSError' exception in case certain job cache path doesn't exist

[meaksh]

What does this PR do?

This PR fixes an issue that may occur when exploring the local salt jobs cache.

If master has certain workload, or salt job cache is cleaned while i.a. jobs.list_jobs is running, then we're going to get an OSError exception due non-existing path.

salt/run/20170212133049657941/new	{
    "_stamp": "2017-02-12T11:30:50.125548", 
    "fun": "runner.jobs.list_jobs", 
    "jid": "20170212133049657941", 
    "user": "admin"
}
salt/run/20170212133049657941/ret	{
    "_stamp": "2017-02-12T11:30:51.183745", 
    "fun": "runner.jobs.list_jobs", 
    "jid": "20170212133049657941", 
    "return": "Exception occurred in runner jobs.list_jobs: Traceback (most recent call last):\n  File \"/usr/lib/python2.7/site-packages/salt/client/mixins.py\", line 348, in low\n    data['return'] = self.functions[fun](*args, **kwargs)\n  File \"/usr/lib/python2.7/site-packages/salt/runners/jobs.py\", line 333, in list_jobs\n    ret = mminion.returners['{0}.get_jids'.format(returner)]()\n  File \"/usr/lib/python2.7/site-packages/salt/returners/local_cache.py\", line 343, in get_jids\n    for jid, job, _, _ in _walk_through(_job_dir()):\n  File \"/usr/lib/python2.7/site-packages/salt/returners/local_cache.py\", line 69, in _walk_through\n    for final in os.listdir(t_path):\nOSError: [Errno 2] No such file or directory: '/var/cache/salt/master/jobs/32'\n", 
    "success": false, 
    "user": "admin"
}

The _walk_through loop is not checking the actual existence of the t_path before accessing it. We should ensure that such path is existing just before calling os.listdir().

Same is already done for the final job file just before open it.

Tests written?

No

/cc @moio

[cachedout]

Very good. Thanks, @meaksh