Endless loop on a non-existent path

[fATwaer]

qscamel will try to list a non-existent path in the local file system.
the task.yaml is shown below:

type: copy

source:
  type: fs
  path: /path/to/non_existent_path

destination:
  type: fs
  path: /path/to/destination

the running result is:

ERRO[2019-02-25T12:50:03+08:00] List object / failed for open /path/to/non_existent_path: no such file or directory. 
INFO[2019-02-25T12:50:03+08:00] There are not finished directory objects.    
INFO[2019-02-25T12:50:03+08:00] Start listing job /.                         
ERRO[2019-02-25T12:50:03+08:00] Src list failed for open /path/to/non_existent_path: no such file or directory. 
ERRO[2019-02-25T12:50:03+08:00] List object / failed for open /path/to/non_existent_path: no such file or directory. 
INFO[2019-02-25T12:50:03+08:00] There are not finished directory objects.    
INFO[2019-02-25T12:50:03+08:00] Start listing job /.                         
ERRO[2019-02-25T12:50:03+08:00] Src list failed for open /path/to/non_existent_path: no such file or directory. 
ERRO[2019-02-25T12:50:03+08:00] List object / failed for open /path/to/non_existent_path: no such file or directory. 
INFO[2019-02-25T12:50:03+08:00] There are not finished directory objects.    
INFO[2019-02-25T12:50:03+08:00] Start listing job /.                         
ERRO[2019-02-25T12:50:03+08:00] Src list failed for open /path/to/non_existent_path: no such file or directory. 
ERRO[2019-02-25T12:50:03+08:00] List object / failed for open /path/to/non_existent_path: no such file or directory. 
INFO[2019-02-25T12:50:03+08:00] There are not finished directory objects.    
INFO[2019-02-25T12:50:03+08:00] Start listing job /.                         
ERRO[2019-02-25T12:50:03+08:00] Src list failed for open /path/to/non_existent_path: no such file or directory. 
ERRO[2019-02-25T12:50:03+08:00] List object / failed for open /path/to/non_existent_path: no such file or directory. 
INFO[2019-02-25T12:50:03+08:00] There are not finished directory objects.    
INFO[2019-02-25T12:50:03+08:00] Start listing job /.                         
ERRO[2019-02-25T12:50:03+08:00] Src list failed for open /path/to/non_existent_path: no such file or directory. 
ERRO[2019-02-25T12:50:03+08:00] List object / failed for open /path/to/non_existent_path: no such file or directory. 
...

[fATwaer]

client method Writable() isn't implement.

[fATwaer]

7946c48#diff-d1c2fd5bd8f1b676e9ba7755964ab5a3R114 fix up this problem, by adding a checking step