(check_file): Report error right away if I/O fails,

so that the proper errno value is used.
(check_file): Check for ferror (stdout) even if ostream == stdout.
(check_file): Don't report bogus errno value
after ferror discovers an output error.  We don't know the proper
errno value, since it might have been caused by any of a whole
bunch of calls, and it might have been trashed in the meantime.
Fixing this problem will require much more extensive changes;
in the meantime just say "write error".

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