Enhance database connection handling

[rafal-szypulka]

Proposed fix for issue: #252

1. If a database connection fails, it will log the error and skip that database.
2. If at least one database connects successfully, the exporter will start and expose metrics for the connected databases.
3. If no databases connect successfully, the exporter will exit.

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[markxnelson]

Hi @rafal-szypulka thank you very much for your contribution, we will take a look and get back to you soon. Please note that we are required to have contributors sign the agreement before we can merge code. Best regards, @markxnelson