Improve error message when Milestone does not exist in redmine.

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```

Start in .github/workflows/scripts/release.py, especially validate_and_update_redmine_data at lines 61-66, and read the traceback to understand the missing-milestone path. Confirm the behavior when redmine_issue.fixed_version is unavailable. Done means the script reports "Couldn't find milestone x.y.z" instead of producing the ResourceAttrError and UnboundLocalError traceback.