Introduce send_strerror().

There are many places in various command handlers where the error path
contains code like

	sc_send_va_buffer(&cc->scc, "%s\n", para_strerror(-ret));

to send an error message to the client.  This patch simplifies all
these places by introducing the public function send_strerror() which
takes a command context structure and an error code, and executes the
above statement. All places where this was open-coded are changed
to call the new function instead.