Fixed in hg 6356:c26b9ce33ae3.

One glitch is that the error technically should throw an
OptionValueError. However there is no good way to get the
object required to use that exception because the test
has to occur outside of the Options application framework.

So I just throw a ValueError like the former implementation
in indexer_xapian.py.

This version does exit on startup rather than on first access so
a win nonetheless.