SwingStatusBar: fix bug in warning status messages

The problem was introduced in f81648f65ce235775487c94abaf8aa27b6da0fef.
A StatusEvent is fired with "" message, which the SwingStatusBar
overzeaolously converts into the status bar's default version blurb
using StatusService#getStatusMessage. But if the status event is a
warning, we should not do that.

Fixes #50.