graylog initialization error handling

[dslarm]

When using the graylog framework, if the pygelf module is not installed, the error is "could not initialize the graylog handler; ignoring". No other error is flagged.

I spent a while 'fixing' graylog server firewalls - assuming the error was elsewhere before looking at the source of core/logging.py

It would be helpful to report the error that module cannot be imported - specifically, in _create_graylog_handler - the ImportError exception handler should log before it returns None.

[vkarak]

Hi @dslarm, thanks for the suggestion! It definitely make sense to have a more precise message there.