Just log when skipping a large file during save

If bup-save --smaller= is specified, the user *expects* large files
to be skipped.  Having bup report errors and return exitcode 1 in this
case hides real errors.  Instead, log it when running verbosely, but
not otherwise.

Signed-off-by: Nick Alcock <nick.alcock@oracle.com>
[rlb@defaultvalue.org: adjust commit summary; simplify opt.verbose >= 1
 to opt.verbose]
Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>