provide proper error message when image is not found on singularity hub

[yarikoptic]

ATM

$> datalad containers-add blah -u shub://ReproNim/containers:bids-validator--non-existent 
[ERROR  ] 'image' [containers_add.py:_resolve_img_url:42] (KeyError)

traceback/details:

$> datalad --dbg containers-add bids-validator -i images/bids/bids-validator--1.3.1.sing --update --call-fmt '{img_dspath}/scripts/singularity_cmd run {img} {cmd}' -u shub://ReproNim/containers:bids-validator--1.3.1 
Traceback (most recent call last):
  File "/usr/bin/datalad", line 8, in <module>
    main()
  File "/usr/lib/python3/dist-packages/datalad/cmdline/main.py", line 494, in main
    ret = cmdlineargs.func(cmdlineargs)
  File "/usr/lib/python3/dist-packages/datalad/interface/base.py", line 628, in call_from_parser
    ret = list(ret)
  File "/usr/lib/python3/dist-packages/datalad/interface/utils.py", line 435, in generator_func
    result_renderer, result_xfm, _result_filter, **_kwargs):
  File "/usr/lib/python3/dist-packages/datalad/interface/utils.py", line 529, in _process_results
    for res in results:
  File "/usr/lib/python3/dist-packages/datalad_container/containers_add.py", line 222, in __call__
    imgurl = _resolve_img_url(url)
  File "/usr/lib/python3/dist-packages/datalad_container/containers_add.py", line 42, in _resolve_img_url
    url = shub_info['image']
KeyError: 'image'

> /usr/lib/python3/dist-packages/datalad_container/containers_add.py(42)_resolve_img_url()
-> url = shub_info['image']
(Pdb) p shub_info
{'detail': 'Not found.'}

I think we might want to define a proper exception (ImageNotFoundError base class, and SingularityHubImageNotFoundError) to be raised in corresponding spot(s), and then caught/reported consistently across backends with an more informative error message error, e.g.
"Failed to resolve image url for shub://ReproNim/containers:bids-validator--non-existent . Response was: 'Not found'" or alike

[yarikoptic]

another error message to expect (I added a print for the manifest there before the actual "logic" kicks in):

$> singularity pull shub://ReproNim/containers:bids-freesurfer--v6.0.1-5
{u'detail': u'Request was throttled. Expected available in 17837 seconds.'}
Traceback (most recent call last):
  File "/usr/lib/x86_64-linux-gnu/singularity/python/pull.py", line 74, in <module>
    main()
  File "/usr/lib/x86_64-linux-gnu/singularity/python/pull.py", line 66, in main
    layerfile=LAYERFILE)
  File "/usr/lib/x86_64-linux-gnu/singularity/python/shub/main.py", line 87, in PULL
    image_name = get_image_name(manifest)
  File "/usr/lib/x86_64-linux-gnu/singularity/python/shub/api.py", line 242, in get_image_name
    return get_default_name(manifest)
  File "/usr/lib/x86_64-linux-gnu/singularity/python/shub/api.py", line 262, in get_default_name
    version = manifest['branch']
KeyError: 'branch'