Fix generate error display when your arity is not respected.

[gaelL]

My provision stop on this line (no error but stop) :

./provision.sh ...
+ .../config-tools/generate.py 0 ...config-tools/global.yml ...config-tools/config.tmpl ... > $TOP/etc/config-tools/config

This fix change printed error to stderr :

Not the expected arity (3+2n) for controller: 0

The result is :

generate.py 0 ... > /dev/null
Not the expected arity (3+2n) for controller: 0

[fredericlepied]

Thx Gael