As I did on app/, finalizing an output stream also implicitly flushes
and closes it. Hence if an export ended with an error, we'd end up with
incomplete data file (possibly overwriting a previously exported image).
Only 2 plug-ins I haven't fixed yet are file-tiff-io and file-gif-save.
The later one don't even clean up its memory (which somehow is good here
as at least the output stream is never finalized hence sane files are
not overwritten in case of errors). As for the former (TIFF plug-in), it
doesn't even seem to have any error control AFAICS, apart from printing
error messages on standard error output.