sum,cksum: consistently emit detailed write errors

sum,cksum: consistently emit detailed write errors

Previously the string from strerror would only be printed for some
algorithms:

    $ cksum -a sha2 -l 256 /dev/null > /dev/full
    cksum: write error: No space left on device
    $ cksum -a sysv /dev/null > /dev/full
    cksum: write error

This patch fixes it so that the error information is always printed:

    $ ./src/cksum -a sha2 -l 256 /dev/null > /dev/full
    cksum: write error: No space left on device
    $ ./src/cksum -a sysv /dev/null > /dev/full
    cksum: write error: No space left on device

* src/cksum.c (output_file): Don't check for a write error here.
(main): Check for it here instead.
* tests/misc/io-errors.sh: Add a 'cksum' invocation for each supported
algorithm.
https://github.com/coreutils/coreutils/issues/258