User-friendlier mkdir error messages

Instead having this:

    Traceback (most recent call last):
      File "./osmose_run.py", line 767, in <module>
        err_code |= run(country_conf, logger, options)
      File "./osmose_run.py", line 474, in run
        os.makedirs(conf.dir_tmp)
      File "/usr/lib/python2.7/os.py", line 150, in makedirs
        makedirs(head, mode)
      File "/usr/lib/python2.7/os.py", line 150, in makedirs
        makedirs(head, mode)
      File "/usr/lib/python2.7/os.py", line 150, in makedirs
        makedirs(head, mode)
      File "/usr/lib/python2.7/os.py", line 157, in makedirs
        mkdir(name, mode)
    OSError: [Errno 13] Permission denied: '/data'

We now have this:

    [Errno 13] Permission denied: '/data'
    Check 'dir_work' in modules/config.py and its permissions
Merge remote-tracking branch 'naoliv/mkdir'