Better error when charm download fails

[tvansteenburgh]

(venv)mbruzek@warhorse:~/workspace/bundles/kubernetes-cluster$ bundletester -F -l DEBUG -e amazon -v -b bundle.yaml
DEBUG:deployer.charm:Cache dir /home/mbruzek/.juju/.deployer-store-cache/cs_~kubernetes_trusty_kubernetes-6
DEBUG:deployer.charm:Cache dir /home/mbruzek/.juju/.deployer-store-cache/cs_~kubernetes_trusty_kubernetes-master-8
DEBUG:deployer.charm:Cache dir /home/mbruzek/.juju/.deployer-store-cache/cs_~kubernetes_trusty_flannel-docker-4
DEBUG:deployer.charm:Retrieving store charm cs:~kubernetes/trusty/flannel-docker-4
Traceback (most recent call last):
  File "/tmp/venv/bin/bundletester", line 9, in <module>
    load_entry_point('bundletester==0.5.4', 'console_scripts', 'bundletester')()
  File "/tmp/venv/local/lib/python2.7/site-packages/bundletester/tester.py", line 89, in main
    suite = spec.SuiteFactory(options, options.testdir)
  File "/tmp/venv/local/lib/python2.7/site-packages/bundletester/spec.py", line 252, in SuiteFactory
    suite.find_suite()
  File "/tmp/venv/local/lib/python2.7/site-packages/bundletester/spec.py", line 123, in find_suite
    self.options.deployment)
  File "/tmp/venv/local/lib/python2.7/site-packages/bundletester/utils.py", line 21, in fetch_deployment
    deployment.fetch_charms()  # update=True)
  File "/tmp/venv/local/lib/python2.7/site-packages/deployer/deployment.py", line 188, in fetch_charms
    charm.fetch()
  File "/tmp/venv/local/lib/python2.7/site-packages/deployer/charm.py", line 154, in fetch
    self._fetch_store_charm()
  File "/tmp/venv/local/lib/python2.7/site-packages/deployer/charm.py", line 195, in _fetch_store_charm
    ufh = urllib2.urlopen(store_url)
  File "/usr/lib/python2.7/urllib2.py", line 154, in urlopen
    return opener.open(url, data, timeout)
  File "/usr/lib/python2.7/urllib2.py", line 437, in open
    response = meth(req, response)
  File "/usr/lib/python2.7/urllib2.py", line 550, in http_response
    'http', request, response, code, msg, hdrs)
  File "/usr/lib/python2.7/urllib2.py", line 475, in error
    return self._call_chain(*args)
  File "/usr/lib/python2.7/urllib2.py", line 409, in _call_chain
    result = func(*args)
  File "/usr/lib/python2.7/urllib2.py", line 558, in http_error_default
    raise HTTPError(req.get_full_url(), code, msg, hdrs, fp)
urllib2.HTTPError: HTTP Error 404: Not Found