Check response type in _extract_error_json

The response being passed to _extract_error_json
is assumed to be a requests.Response but can also
be a http.client.HTTPResponse.

This patch adds a try statement where it first tries
for a requests.Response and if not set falls back on
parsing the string body instead of trying to use the
json() helper function in requests.Response.

Change-Id: I61840d284b5d3a249f621822b277c9bd6825140d
Closes-Bug: 1870264

magnumclient/common/httpclient.py

@@ -60,7 +60,10 @@ def _extract_error_json_text(body_json):
 
 def _extract_error_json(body, resp):
     """Return error_message from the HTTP response body."""
+    try:
         content_type = resp.headers.get("Content-Type", "")
+    except AttributeError:
+        content_type = ""
     if content_type.startswith("application/json"):
         try:
             body_json = resp.json()

releasenotes/notes/bug-1870264-1b4f14618b8422f9.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    [`bug 1870264 <https://bugs.launchpad.net/python-magnumclient/+bug/1870264>`_]
+    Fixed bug where the error message from the response could not be parsed.