Use check-handle draft syntax in gdu

[dundee]

Some notes from the rewriting to check-handle syntax:

• it removes a lot of boiler-plate code - the handling of errors is now almost a half
• it's little bit harder to read and understand (mostly when explicit handler is used)
• one has to think on every occasion if check can be used (because control cannot resume if check fails)

Both 2 and 3 are probably just matter of time. It's a new concept, brain has to adapt to it :)

The removal of unnecessary error handling code and thus more meaningful code in general are probably worth all the cons :+1: