Rework "bup fsck" output; allow less verbosity.

Only print debug messages when verbose > 1 (i.e. -vv).

Use log() to print error messages to stderr instead of stdout.

When verbose level is at least 1, print a single descriptive line to
stdout.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
fsck-cmd.py: add par2_exists as a do_pack() argument.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
fsck-cmd.py: simplify do_pack() else clause.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>