libssh2: send the correct CURLE error code on scp file not found

That also updates tests to expect the right error code

libssh2 back-end returns CURLE_SSH error if the remote file
is not found. Expect instead CURLE_REMOTE_FILE_NOT_FOUND
which is sent by the libssh backend.

Signed-off-by: Nikos Mavrogiannopoulos <nmav@redhat.com>