Add --debug flag and handle unknown errors gracefully

Previously, uncaught exceptions resulted in a traceback from
python. Now, they result in a a "fatal error" with a message about the
--debug flag. When the --debug flag is set, uncaught exceptions result
in a traceback from python in order to aid in debugging.
'init' command: handle file permissions gracefully

Failure to create git-raw's directory structure and config file are now
handled gracefully by the 'init' command. It now prints a meaningful
message as opposed to previously when errors resulted in uncaught
exceptions.