Adding basic error handling to GUI

Sometimes blight silently throws exception, leaving GUI in inconsistent state and without any notices, so you might lose messages you are going to send.

This PR adds handling of top-level uncaught exceptions to GUI, showing a dialog box with some context.
Debugging output to console is preserved.

For now exceptions generated during the initialization are not handled, because we need to split it to multiple sequential calls (like read-data(), connect(), init-gui(), bootsrap() etc).

A screenshot
![image](http://dump.bitcheese.net/images/upavupu/blight-error.png)