pkt-line: do not report packet write errors twice

pkt-line: do not report packet write errors twice

On write() errors, packet_write() dies with the same error message that
is already printed by its callee, packet_write_gently(). This produces
an unnecessarily verbose and repetitive output:

error: packet write failed
fatal: packet write failed: <strerror() message>

In addition to that, packet_write_gently() does not always fulfill its
caller expectation that errno will be properly set before a non-zero
return. In particular, that is not the case for a "data exceeds max
packet size" error. So, in this case, packet_write() will call
die_errno() and print an strerror(errno) message that might be totally
unrelated to the actual error.

Fix both those issues by turning packet_write() and
packet_write_gently() into wrappers to a common lower level function
that doesn't print the error message, but instead returns it on a buffer
for the caller to die() or error() as appropriate.

Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Junio C Hamano <gitster@pobox.com>