handle 504s in tile downloads

[Andrew L. Johnson]

Traceback (most recent call last):
File "label_chunks_cnn.py", line 109, in
odn.download_tiles()

raise HTTPError(req.full_url, code, msg, hdrs, fp)

urllib.error.HTTPError: HTTP Error 504: Gateway Time-out

[Andrew L. Johnson]

wont fix stuff for experiment 1, working on experiment 2