Print out response body when a 500 Internal Server Error is received from the pushgateway

I accidentally sent malformed data to a push gateway and got back a 500 Internal Server Error. The body of the response had information about the malformed data but all that was printed was the fact that I got a 500 Internal Server Error.

Example code:

import prometheus_client

reg = prometheus_client.CollectorRegistry()
c1 = prometheus_client.Counter("foo", "bar", labelnames=["baz", "stuff"], registry=reg)
c1.labels({"baz": 1, "stuff": 2}).inc(1)
c2 = prometheus_client.Counter("foo", "bar", labelnames=["baz", "stuff"], registry=reg)
c2.labels({"baz": 3, "stuff": 4}).inc(2)
prometheus_client.push_to_gateway('localhost:8000', job='example', registry=reg)

Output:

Traceback (most recent call last):
  File "test.py", line 8, in <module>
    prometheus_client.push_to_gateway('localhost:8000', job='example', registry=reg)
  File "/usr/local/lib/python2.7/dist-packages/prometheus_client/exposition.py", line 110, in push_to_gateway
    _use_gateway('PUT', gateway, job, registry, grouping_key, timeout)
  File "/usr/local/lib/python2.7/dist-packages/prometheus_client/exposition.py", line 144, in _use_gateway
    resp = build_opener(HTTPHandler).open(request, timeout=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 500: Internal Server Error

Start in prometheus_client/exposition.py, at push_to_gateway and _use_gateway, using the malformed-data example to reproduce the HTTP 500 response. Check how the response error is currently surfaced and verify that the push failure includes the response body while preserving the existing error details.