Catch PyPump network errors and make them pretty

[moggers87]

p should catch network errors and shorten them to something more user-friendly

Example proxy error:

Traceback (most recent call last):
  File "/home/moggers/workspace/p/env/bin/p", line 5, in <module>
    pkg_resources.run_script('p==0.1', 'p')
  File "/home/moggers/workspace/p/env/lib/python2.7/site-packages/pkg_resources.py", line 540, in run_script
    self.require(requires)[0].run_script(script_name, ns)
  File "/home/moggers/workspace/p/env/lib/python2.7/site-packages/pkg_resources.py", line 1462, in run_script
    exec_(script_code, namespace, namespace)
  File "/home/moggers/workspace/p/env/lib/python2.7/site-packages/pkg_resources.py", line 41, in exec_
    exec("""exec code in globs, locs""")
  File "<string>", line 1, in <module>
  File "/home/moggers/workspace/p/env/lib/python2.7/site-packages/p-0.1-py2.7.egg/EGG-INFO/scripts/p", line 634, in <module>

  File "/home/moggers/workspace/p/env/lib/python2.7/site-packages/p-0.1-py2.7.egg/EGG-INFO/scripts/p", line 483, in inbox

  File "build/bdist.linux-i686/egg/pypump/models/feed.py", line 96, in next
  File "build/bdist.linux-i686/egg/pypump/models/feed.py", line 82, in __next__
  File "build/bdist.linux-i686/egg/pypump/models/feed.py", line 71, in _build_cache
  File "build/bdist.linux-i686/egg/pypump/models/feed.py", line 163, in _request
  File "build/bdist.linux-i686/egg/pypump/pypump.py", line 322, in request
pypump.exception.PyPumpException: Request Failed to https://microca.st/api/proxy/UL6BQ7pZRG-cpV3qnsZD1Q (response: {"error":"Unable to retrieve proxy data"} | status: 500)