Sometimes Jira returns dicts as "errors" instead of simple strings.
For example, when a user specifies a field that cannot be set, Jira
returns a dict with the field name as a key and the error message as the
value.
In the rare case that we have both a "errorMessages" list and an
"errors" dict, when we combine those values later with join(), Python
raises a TypeError.
Transform each individual error message into a string, and then join()
the list of strings.
