When calling `logger.removeHandler(h)` with one of our custom
`NullHandler` instances, things crashed.

Normal errors should be displayed by default, even if they are not
catastrophic.

As we can only increase verbosity with commandline flags, we
should set the least verbose log level by default.

Tell at least, what level can be set.

When we set logging levels, then handlers must support this.

Add a common place where components can write messages to. Any
userinterface can decide, what messages should be visible
(depending on their severity).