We can still get useful error messages out of d.Try without
specifically wrapping every single thing that might throw a
noms UsageError, so do so. The code is cleaner.
Towards issue #176
Introduces the notion of debug 'expectations', analogous to the
'checks' that we already have. d.Exp provides the same API as d.Chk,
but expectation violations can be caught using d.Try().
Toward issue #176
