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

[Alice BRENON]

Hello,

I triggered a confusing behaviour from the opam importer trying to
import package iter 1.2.1 today on a Guix System install.

The package iter is missing a "synopsis" field as can be seen on
https://opam.ocaml.org/packages/iter/ , which when I tried

guix import opam iter

yielded the following backtrace:

Backtrace:
           8 (primitive-load "/home/alice/.config/guix/current/bin/g…")
In guix/ui.scm:
   2185:7  7 (run-guix . _)
  2148:10  6 (run-guix-command _ . _)
In guix/scripts/import.scm:
   120:11  5 (guix-import . _)
In guix/scripts/import/opam.scm:
   104:23  4 (guix-import-opam . _)
In guix/utils.scm:
    752:8  3 (call-with-temporary-output-file _)
In guix/import/opam.scm:
   337:34  2 (_ _ _)
In srfi/srfi-1.scm:
   460:18  1 (fold #<procedure 7f3baca56fe0 at guix/import/opam.scm…> …)
In guix/import/opam.scm:
   193:15  0 (_ _ _)

guix/import/opam.scm:193:15: Throw to key `match-error' with args
`("match" "no matching pattern" string-pat)'.


the final error is raised l.193 of guix/import/opam.scm because
metadata-ref supports various types for a metadata field, but not the
lack of it. As discussed with Maxime Devos on the IRC channel, it would
be helpful to either allow the import of a package with a missing field
(possibly filling it in the output scheme code for the imported package
with some bad value requiring the user to fill it and causing any build
to crash until replaced properly) or at least to handle that missing
field with a more explicit error message than the above backtrace
(something like "Can't import that package because it's missing such or
such field").

Thanks,

Alice BRENON