Remove null handler

[mdickinson]

Envisage currently adds a logging NullHandler to the "envisage" logger. That has the (deliberate, intended) effect of suppressing log messages from Envisage at WARNING and ERROR level if no handlers have been configured for the root logger.

I suspect that that isn't really what we want, and risks hiding useful information: if something inside Envisage is logging at WARNING or ERROR level, that likely indicates that there's an action to be taken by the user of Envisage.

I'd suggest:

• reviewing Envisage's uses of logger.warning and logger.error to see if any of those warnings and errors should be downgraded to logger.info or lower, and whether the remaining warnings and errors are reasonably actionable by Envisage users
• removing the NullHandler