sub-process: print the cmd when a capability is unsupported

In handshake_capabilities() we use warning() when a capability is not supported, so the exit code of the function is 0 and no further error is shown. This is a problem because the warning message doesn't tell us which subprocess cmd failed. On the contrary if we cannot write a packet from this function, we use error() and then subprocess_start() outputs: initialization for subprocess '<cmd>' failed so we can know which subprocess cmd failed. Let's improve the warning() message, so that we can know which subprocess cmd failed. Helped-by: Lars Schneider <larsxschneider@gmail.com> Signed-off-by: Christian Couder <chriscool@tuxfamily.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>