dd: fix a bug where we don't report write errors
testsuite: small cleanup

full_write_or_warn                                    38      40      +2
write_and_stats                                       66      67      +1
dd_main                                             1358    1335     -23
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(add/remove: 0/0 grow/shrink: 2/1 up/down: 3/-23)             Total: -20 bytes

2 files changed