Use captureTopConstraints in TcRnDriver calls
Trac Trac #16376 showed the danger of failing to report an error
that exists only in the unsolved constraints, if an exception
is raised (via failM).
Well, the commit 5c1f268e (Fail fast in solveLocalEqualities)
did just that -- i.e. it found errors in the constraints, and
called failM to avoid a misleading cascade.
So we need to be sure to call captureTopConstraints to report
those insolubles. This was wrong in TcRnDriver.tcRnExpr and
As a result the error messages from test T13466 improved slightly,
a happy outcome.