busybox

libbb/xwrite: print errno on "short write" errors

Lauri Kasanen:
:: Over at TinyCore, we receive a huge number of questions of the type "I
:: got "short write", what does it mean?". Mostly for the rpi port and when
:: using bb wget.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>