journalctl: be smarter about journal error checks

There are many ways in which we can get those checks wrong, so it is
better to warn and then error out on a real access failure.

The error messages are wrapped to <80 lines, because their primary
use is to be displayed in the terminal, and it is easier to read them
this way. Reading them in the journal can be a bit trickier, but
this is a bug in logs-show.c.