Review all log.exception() calls

Actually, log.exception() is often used with an Exception as argument. This is wrong and we end on different errors when reporting these lines.

It must be used with an sentence, and the exception itself will be automatically appened to the log (using the right encoding).

To resume, log.exception(message) is just an alias for log.error(message, exc_info=True).

Sources:

• http://stackoverflow.com/a/31141546
• https://docs.python.org/2/library/logging.html#logging.Logger.exception