chore: Error handling for decrypt()

[DafyddLlyr]

What does this PR do?

• Wraps decrypt() in a try/catch
• Currently, if decryption fails, an unhandled exception brings down Express
• This now ensures it's handled gracefully, and the consumer needs to check the existence of they keys (the function still maintains it's signature of returning string | undefined for a secret)

To test

• Make a secret invalid, e.g. populate encrypted GovPay token as plaintext string "abc123"
• Try to make payment
• Graceful error handling in application, Docker logs capture issue
• Example flow here on Pizza with an invalid GovPay key - https://3848.planx.pizza/epsom-and-ewell/test-pay-failure/published?analytics=false

[jessicamcinchak]

Perfect thanks !