bug#49827: Error message for missing synopsis in opam importer

[Julien Lepiller]

Hi,

First, today when running guix import opam iter, I get a synopsis, and
#f as the description because the field is missing.

I also pushed a small patch to master, as
24aa7b3c21309b63cc6e8e18d6417d2cddccf6c6, that ensures that, when the
field exists but contains unknown data, we also return #f instead of a
match error that produces the ugly backtrace you saw.

Thanks for the report, closing :)