fix(show-refs): skip remote-tracking refs without a configured remote

[SAY-5]

Opening Show Refs panics with Couldn't create screen: GetRemote(...) when a remote-tracking ref exists whose remote cannot be determined, e.g. a leftover refs/remotes/conflict-upstream/main from a conflicted rebase. create_remotes_sections propagated the branch_remote_name error and aborted the whole screen. Now such refs are skipped so the rest of the refs still render.

Fixes #556.

Checklist

• The modified code has some test-coverage (where applicable).
• New unreleased features/fixes/styling and performance improvements are documented via git: feat: / fix: / style: / perf:. Or e.g. perf(highlighting):.
  See https://github.com/altsem/gitu/blob/master/docs/dev-tooling.md