Catch illegal boolean values in hgrc nicely.
With hg 0.9.3 the section and parameter name wasn't printed,
with hg 0.9.4 there was a traceback for ValueError.
1 files updated, 0 files merged, 0 files removed, 0 files unresolvedremoving babort: update spans branches, use 'hg merge' or 'hg update -C' to lose changes