Handle warnings correctly in all cases

It has been a long-standing omission to ensure correct error handling for warnings.
A widely unknown feature of Python is the ability to turn any warning into
a real exception. This can be triggered by passing the -Werror flag on Python startup
or the environment variable PYTHONWARNINGS=error.

The needed action is

• Make the current Shiboken::warning function correctly ignore errors,
• Write a better function that handles warnings in a conforming way,
• Rewrite places with warning usage to be aware of exceptions.