b4-wrapper-poky.py: output errors to stderr

b4-wrapper-poky.py: output errors to stderr

Print error messages to stderr instead of stdout.

The commands run as part of send-auto-cc-cmd and send-auto-to-cmd b4
commands will make b4 raise an Exception if they return a non-zero
return code and it will only print the content of stderr before that.

Because print defaults to stdout, b4 would raise the exception and not
tell the user why. This commit should now provide a hint to the user
about what went wrong.

Reported-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>