[ISSUE-1242] prevented weird exceptions related to integer handling (#1244)

if keys or values in the response json are `int`s, the `k + ": " + v`
will fail because of an `int` is not a `str`.  In order to avoid this,
we can either use `%`-formatting, `.format` formatting, or f-strings.
We chose `.format` for compatability, but regardless of which direction, the
issue should be addressed one of those ways.

Author: 2ps

atlassian/rest_client.py

@@ -481,7 +481,7 @@ def raise_for_status(self, response):
             try:
                 j = response.json()
                 if self.url == "https://api.atlassian.com":
-                    error_msg = "\n".join([k + ": " + v for k, v in j.items()])
+                    error_msg = "\n".join(["{}: {}".format(k, v) for k, v in j.items()])
                 else:
                     error_msg_list = j.get("errorMessages", list())
                     errors = j.get("errors", dict())