perf(bwcompat): do not print the traceback for 4xx HTTP errors
These errors are related to the client and do not needs to be visible in the logging output.
For example, when the CubicWeb instance if not public and the user try to access a ressource, the instance will raise an HTTPUnauthorized exception. This traceback do not have to be visible, since this is not a CubicWeb error, but a valid behavior with Pyramid.