Fix print function calls in rosbag_main.py

- The print call in ProgressMeter prints 'None' and a newline to the
  screen on every update which looks terrible in tools like 'rosbag
  compress'.  Change to not output a newline and still flush the output.
- Several calls to 'print', but it's now a function so it needs to be
  evaluated to print a newline.
Fix missing newlines in rosbag error messages

In rosbag_main.py, switch all calls to sys.stderr.write(args) to
print(args, file=sys.stderr).  This makes the error messages more
readable.