cmd/fsck-cmd.py: Append newline after quoted par2 output

cmd/fsck-cmd.py: Append newline after quoted par2 output

With a recent `par2` installed, `bup fsck -v` was printing messages
like this:

  Unexpected par2 error output
  ''Assuming par2 supports parallel processing

when there was no error output and the exit status was zero.  A
previous commit 19f9faeb0055dadb7f76a953d51acec8373c6edb eliminated
the spurious reporting; now make sure we print a newline after the
par2 output whenever there actually is an error.

Signed-off-by: Christian Cornelssen <ccorn@1tein.de>
[rlb@defaultvalue.org: add information from the pr to commit message]
Reviewed-by: Rob Browning <rlb@defaultvalue.org>