[track] Comparison image retrieval for non-OSSOS images

[mtbannister]

Comparison image requests for non-OSSOS images fail because the cutout url constructor is only looking at /dbimages. Can we have a fallback pointing to public MegaCam imaging?

Traceback (most recent call last):
File "/Users/bannisterm/miniconda2/envs/astro/lib/python2.7/site-packages/ossos/downloads/cutouts/source.py", line 423, in retrieve_comparison_image
SkyCoord(ref_ra, ref_dec), radius)
File "/Users/bannisterm/miniconda2/envs/astro/lib/python2.7/site-packages/ossos/storage.py", line 763, in ra_dec_cutout
resp = requests.get(url, params, auth=vospace.authentication)
File "/Users/bannisterm/miniconda2/envs/astro/lib/python2.7/site-packages/ossos/storage.py", line 99, in get
resp.raise_for_status()
File "/Users/bannisterm/miniconda2/envs/astro/lib/python2.7/site-packages/requests/models.py", line 862, in raise_for_status
raise HTTPError(http_error_msg, response=self)
HTTPError: 404 Client Error: Not Found for url: http://www.canfar.phys.uvic.ca/vospace/auth/nodes/OSSOS/dbimages/2076144/2076144p.fits?cutout=CIRCLE+ICRS+220.06808944+-16.2636077039+0.00538634363027&view=cutout