(GITHUB-89) Do not throw ValueError on authentication error during POST

[jhaegg]

Fix for issue #89

Before:

>>> import librato
>>> conn = librato.connect("user", "invalidcredential")
>>> conn.list_metrics()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../librato/__init__.py", line 222, in list_metrics
    resp = self._mexe("metrics", query_props=query_props)
  File ".../librato/__init__.py", line 191, in _mexe
    resp_data, success, backoff = self._process_response(resp, backoff)
  File ".../librato/__init__.py", line 171, in _process_response
    raise exceptions.get(resp.status, resp_data)
librato.exceptions.Unauthorized: [401] request: Authorization Required

>>> conn.submit("gauge_1", 1, description="desc 1")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../librato/__init__.py", line 246, in submit
    self._mexe("metrics", method="POST", query_props=payload)
  File ".../librato/__init__.py", line 191, in _mexe
    resp_data, success, backoff = self._process_response(resp, backoff)
  File ".../librato/__init__.py", line 167, in _process_response
    resp_data = json.loads(body.decode(_getcharset(resp)))
  File "/usr/lib64/python3.5/json/__init__.py", line 319, in loads
    return _default_decoder.decode(s)
  File "/usr/lib64/python3.5/json/decoder.py", line 339, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib64/python3.5/json/decoder.py", line 357, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

After:

>>> import librato
>>> conn = librato.connect("user", "invalidcredential")
>>> conn.list_metrics()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../python-librato/librato/__init__.py", line 219, in list_metrics
    resp = self._mexe("metrics", query_props=query_props)
  File ".../python-librato/librato/__init__.py", line 188, in _mexe
    resp_data, success, backoff = self._process_response(resp, backoff)
  File ".../python-librato/librato/__init__.py", line 168, in _process_response
    raise exceptions.get(resp.status, resp_data)
librato.exceptions.Unauthorized: [401] request: Authorization Required

>>> conn.submit("gauge_1", 1, description="desc 1")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../python-librato/librato/__init__.py", line 243, in submit
    self._mexe("metrics", method="POST", query_props=payload)
  File ".../python-librato/librato/__init__.py", line 188, in _mexe
    resp_data, success, backoff = self._process_response(resp, backoff)
  File ".../python-librato/librato/__init__.py", line 168, in _process_response
    raise exceptions.get(resp.status, resp_data)
librato.exceptions.Unauthorized: [401] Credentials are required to access this resource.

[drio]

Outstanding. I'll merge it later today. Good work. 👏