Cleanup exit

[hkollmann]

According to https://github.com/qooxdoo/qooxdoo-cli/issues/46 i rework the handling:

• the library function do not process an exit any longer. Instead they throw an exception
• Exceptions are handled in the handler function.
• if an exception occurs the exit Code is set to 1
• use qxcli.Utils.UserError where ever it is usefull

[hkollmann]

Thanks! IMHO we should first the Code cleanups before continue with new features..

[cboulanger]

I would say as @johnspackman has already reviewed and the later commits concern only fixes, we can merge this. Thank you very much, @hkollmann, this is the sort of work that is less fun than doing new stuff but soooo important.