do not return the rc from Archiver methods

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