Add new method for output errors

[rodo]

This will help to port to python3 and the print() method.

I do this first PR simple to open the discussion with you, are you agree with this kind of refacto ? I'll always try to not change any behaviour when doing refacto, so the output will be kept as they are actually

[rodo]

It looks like a huge PR but I only add two functions, and always replace the same pattern in the whole file.

Replace

print '--------------------------------------------------------'
print '[ERROR]: ',e,'\n'

by

self.output_error(e)

[rodo]

Ref #50

[usit-gd]

Hello rodo

Thank you very much for your patch.

During the last week I have been testing a new release I have not had time to see this patch. The code is in freeze state until release. It is too late to have it in this release but I will merge to the repository in a few days.

regards