save: catch and log SSH errors

Currently, if the ssh connection to the remote server fails, we get the
server errors, then an ugly traceback that ends up saying "look at the
errors above". It makes it look like the traceback is part of the errors
that we need to look at, when in reality it's just an exception that's
not handled correctly by bup-save.

Signed-off-by: Gabriel Filion <gabster@lelutin.ca>
[rlb@defaultvalue.org: adjust commit summary]
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>