- implement updating exit code based on severity, including modern codes
- extend print_warning with kwargs wc (warning code) and wt (warning type)
- update a global warnings_list with warning_info elements
- create a class hierarchy below BorgWarning class similar to Error class
- diff: change harmless warnings about speed to rc == 0
- delete --force --force: change harmless warnings to rc == 0
Also:
- have BackupRaceConditionError as a more precise subclass of BackupError
If not set, it will default to "legacy" (always return 2 for errors).
This commit only changes the Error exception class and its subclasses.
The more specific exit codes need to be defined via .exit_mcode in the subclasses.
