Return a non-zero exit code with `sys.exit` when updater fails

[n-dusan]

Important for power-users who use taf in conjunction with multiple other commands chained together. e.g. taf repo update && <any_other_command.

Make taf repo update && <any_other_command> execute <any_other_command> only if taf repo update returns a zero exit code.

[renatav]

Closed by merging #550