Fixes <http://bugs.gnu.org/26972>.
* guix/ssh.scm (store-import-channel)[import]: Add 'consume-input'
procedure. Wrap body in 'catch' and 'guard'. Use 'open-remote-pipe'
with OPEN_BOTH instead of 'open-remote-output-pipe'.
(send-files): After the 'channel-send-eof' call, do (read port).
Interpret the result sexp and raise an error condition if needed.