Improve robustness of JSON decoding

If JSON decoding fails, if a field is missing, or if there are no fields, a ConsulException is returned.

consul/callback.py

@@ -72,22 +72,27 @@ def cb(response):
             if response.code == 404:
                 data = None
             else:
-                data = json.loads(response.body)
-                if decode:
-                    for item in data:
-                        if item.get(decode) is not None:
-                            item[decode] = base64.b64decode(item[decode])
-                if is_id:
-                    data = data["ID"]
-                if one:
-                    if data == []:
-                        data = None
-                    if data is not None:
-                        data = data[0]
-                if postprocess:
-                    data = postprocess(data)
+                try:
+                    data = json.loads(response.body)
+                    if decode:
+                        for item in data:
+                            if item.get(decode) is not None:
+                                item[decode] = base64.b64decode(item[decode])
+                    if is_id:
+                        data = data["ID"]
+                    if one:
+                        if data == []:
+                            data = None
+                        if data is not None:
+                            data = data[0]
+                    if postprocess:
+                        data = postprocess(data)
+                except (json.JSONDecodeError, TypeError, KeyError) as e:
+                    raise ConsulException(f"Failed to decode JSON: {response.body} {e}")
             if index:
-                return response.headers["X-Consul-Index"], data
+                if not index in response.headers:
+                    raise ConsulException(f"Missing index header: {response.headers}")
+                return response.headers.get["X-Consul-Index"], data
             return data
 
         return cb