An input/output error indicates a fatal condition, even if it
occurs when closing a file. Awk should not return success on I/O
error, but treat I/O errors as it already treats write errors.
Test case:
$ (trap '' PIPE; awk 'BEGIN { print "hi"; }'; echo "E $?" >&2) | :
awk: i/o error occurred while closing /dev/stdout
source line number 1
E 2
The test case pipes a line into a dummy command that reads no
input, with SIGPIPE ignored so we rely on awk's own I/O checking.
No write error is detected, because the pipe is buffered; the
broken pipe is only detected as an I/O error on closing stdout.
Before this commit, "E 0" was printed (indicating status 0/success)
because an I/O error merely produced a warning. A shell script
was unable to detect the I/O error using the exit status.
