NFC: Add debug level log message when afform submissions catch exceptions

[artfulrobot]

The code comments suggest we don't really know what to do here, previously we did nothing, which was not helpful to a developer trying to figure out why their code did nothing.

Now we emit the exception's message to the log file.

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[mattwire]

Seems ok - similar to #25533 that I added recently - maybe add 'afform' as log prefix ie. \Civi::log('afform')->debug

[artfulrobot]

@mattwire ok, done that.

[eileenmcnaughton]

Merging as it makes sense & @mattwire's feedback is in favour & incorporated already