…odes

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.

- also output modern rc and traceback yes/no
- recursive list of Error subclasses

- _export_tar: remove unneeded call to set_ec
  print_warning() already sets the exit code to EXIT_WARNING.

- msgpack version check: raise Error instead of calling set_ec

previously, this was handled in RPCError handler and always resulted in rc 2.

now re-raise Lock Exceptions locally, so it gives rc 2 (legacy) or 7x (modern).

Users using recently created repos (after borg 1.0.9) or
who followed the upgrade procedure of 1.0.9 or >1.2.4 will
never see this error msg.

So, have it as short as and similar to the ArchiveTAMRequiredError.

- 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

also: use more union operators rather than .union()

this is not needed and getting rid of it makes
the code / behaviour simpler to understand:

if a fatal error is detected, we throw an exception.

if we encounter something warning worthy, we emit and collect the warning.

in a few cases, we directly call set_ec to set the
exit code as needed, e.g. if passing it through
from a subprocess.

also:
- get rid of Archiver.exit_code
- assert that return value of archiver methods is None
- fix a print_warning call to use the correct formatting method

stop directly accessing the variables from other modules.

prefix with underscore to indicate that these shall
only be used within this module and every other user
shall call the respective functions.

if we do multiple calls to Archiver.do_something(),
we need to reset the ec / warnings after each call,
otherwise they will keep growing (in severity, in length).