journalctl: when we fail to open a journal file, print why

When we enumerate journal files and encounter an invalid one, remember
which this, and show it to the user.

Note the possibly slightly surprising logic here: we store only one path
per error code. This means we show all error kinds but not every actual
error we encounter. This has the benefit of not requiring us to keep a
potentially unbounded list of errors with their sources around, but can
still provide a pretty complete overview on the errors we encountered.

Fixes #1669.

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