jira: cast error messages to strings (#707)

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.

changelogs/fragments/707-jira-error-handling.yaml

@@ -0,0 +1,2 @@
+bugfixes:
+- jira - improve error message handling with multiple errors (https://github.com/ansible-collections/community.general/pull/707).