crmMailingAB/services: catch exception, report error and rethrow

[semseysandor]

Overview

If an A/B test's mailing is missing from civicrm_mailings (or can't be loaded for any other reason) opening the relevant A/B test on Mailings >> Manage A/B tests will fail.

Before

WSOD and an empty console, no hint on error.

After

Opening will still fail with a white screen, but there is an error logged to the console, so there is a clue where to start debugging.

[civibot]

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[eileenmcnaughton]

I suspect we could go further than this on the error handling - but I agree it is better than what there is without this PR

@semseysandor are you in contributors.yml yet?

[semseysandor]

Yes, this is only a small step. I've just spent some time debugging and 90% of it was about finding which line fails. Hopefully, this saves someone that time...

@eileenmcnaughton yes, i'm already in 😄