Improve error message when Milestone does not exist in redmine.

[dkliban]

Instead of producing a traceback like below, a proper error message should state "Couldn't find milestone x.y.z".

Repo path: /home/runner/work/pulp-certguard/pulp-certguard
Traceback (most recent call last):
  File ".github/workflows/scripts/release.py", line 61, in validate_and_update_redmine_data
    issue_milestone = redmine_issue.fixed_version
  File "/opt/hostedtoolcache/Python/3.7.10/x64/lib/python3.7/site-packages/redminelib/resources/standard.py", line 152, in __getattr__
    return super(Issue, self).__getattr__(attr)
  File "/opt/hostedtoolcache/Python/3.7.10/x64/lib/python3.7/site-packages/redminelib/resources/base.py", line 197, in __getattr__
    raise exceptions.ResourceAttrError
redminelib.exceptions.ResourceAttrError: Resource doesn't have the requested attribute

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File ".github/workflows/scripts/release.py", line 288, in <module>
    release_commit_sha = create_release_commits(repo, release_version, plugin_path)
  File ".github/workflows/scripts/release.py", line 132, in create_release_commits
    redmine_final_query, issues_to_close, release_version
  File ".github/workflows/scripts/release.py", line 66, in validate_and_update_redmine_data
    redmine.issue.update(redmine_issue.id, fixed_version_id=milestone.id)
UnboundLocalError: local variable 'milestone' referenced before assignment```

[daviddavis]

It might be worth adding a check at the beginning of the release workflow and bailing if there's no milestone.

[stale]

This issue has been marked 'stale' due to lack of recent activity. If there is no further activity, the issue will be closed in another 30 days. Thank you for your contribution!