fsck: allow upgrading fsck warnings to errors

The 'invalid tag name' and 'missing tagger entry' warnings can now be
upgraded to errors by setting receive.fsckobjects to

	+invalid-tag-name,+missing-tagger-entry

Incidentally, the missing tagger warning is now really shown as a warning
(as opposed to being reported with the "error:" prefix, as it used to be
the case before this commit).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>