Send error message on failed attach from debugerserver.

Instead of using a magic return error code from debugserver to
indicate that an attach failed because of SIP being enabled in
RNBRemote::HandlePacket_v, use the extended error reporting that
Pavel added to lldb/lldb-server in https://reviews.llvm.org/D45573


<rdar://problem/39398385> 

llvm-svn: 369990