OSError handling for posix systems.

    Some things that could happen to trigger an OSError:
        - cwd could not exist
            - exc.errno == ENOENT
            - exc.filename == cwd
        - cwd could have permissions that prevent the current user moving to it
            - exc.errno == EACCES
            - exc.filename == cwd
        - cwd could exist but not be a directory
            - exc.errno == ENOTDIR
            - exc.filename == cwd
        - cmd[0] could not exist
            - exc.errno == ENOENT
            - exc.filename == None(?)
        - cmd[0] could exist but have permissions that prevents the current
            user from executing it (executable bit not set for the user)
            - exc.errno == EACCES
            - exc.filename == None(?)

def _handle_posix_error(exc, cwd, cmd):
    """OSError handling for posix systems.

    Some things that could happen to trigger an OSError:
        - cwd could not exist
            - exc.errno == ENOENT
            - exc.filename == cwd
        - cwd could have permissions that prevent the current user moving to it
            - exc.errno == EACCES
            - exc.filename == cwd
        - cwd could exist but not be a directory
            - exc.errno == ENOTDIR
            - exc.filename == cwd
        - cmd[0] could not exist
            - exc.errno == ENOENT
            - exc.filename == None(?)
        - cmd[0] could exist but have permissions that prevents the current
            user from executing it (executable bit not set for the user)
            - exc.errno == EACCES
            - exc.filename == None(?)
    """
    if getattr(exc, 'filename', None) == cwd:
        _handle_posix_cwd_error(exc, cwd, cmd)
    else:
        _handle_posix_cmd_error(exc, cwd, cmd)