(bugfix) archivebot.py: use pywikibot.error

The initial port of archivebot to 'core' left in one call to
parser.error which is used for command line parsing in 'compat'.

Change-Id: Ib1e2841e7cd901a0d298852c29ef5b4025b8852e
Merge "(bugfix) archivebot.py: use pywikibot.error"