Improve error handling & logging on pebble exec timeout

[carlcsaposs-canonical]

Follow up to canonical/mysql-router-k8s-operator#394—investigate if better error handling & logging needed for timeouts

[syncronize-issues-to-jira]

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The internal ticket has been created: https://warthogs.atlassian.net/browse/DPE-6639.

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[syncronize-issues-to-jira]

Thank you for reporting your feedback to us!

The internal ticket has been created: https://warthogs.atlassian.net/browse/DPE-8058.

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