get_wsrep_value doesn't log connection exception

[Adam Dyess]

percona_utils.get_wresp_value(key)

can fail in cursor.execute(...) with an Exception but the logging is

```
unit-mysql-3: 15:53:19 ERROR unit.mysql/3.juju-log Failed to get '%s'
```

changing to

    except Exception as e:
        log("Failed to get key=%s '%s'" % (key, e), ERROR)
        return None

you can get a better log

```
unit-mysql-3: 15:58:33 ERROR unit.mysql/3.juju-log Failed to get key=wsrep_ready '(1146, "Table 'performance_schema.session_status' doesn't exist")'
```

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/charm-percona-cluster/+/850464

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/c/openstack/charm-percona-cluster/+/850464
Committed: https://opendev.org/openstack/charm-percona-cluster/commit/a3f65978a7b6719a901406a31eaf338d9c2f61c3
Submitter: "Zuul (22348)"
Branch: master

commit a3f65978a7b6719a901406a31eaf338d9c2f61c3
Author: Guilherme Maluf Balzana <email address hidden>
Date: Tue Jul 19 16:55:04 2022 +0200

    Fix log message when catching `get_wsrep_value` exception

    `get_wresp_value` can fail in `cursor.execute()` and no log
    message would be written.

    This commit fix and ensure the right message is logged.

    Closes-Bug: #1942936
    Change-Id: Idbb1170bbf43fdecec233137d6581bf8f799baa9