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).
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