[racket] TR error message

At Wed, 22 Sep 2010 08:28:48 -0400,
Matthias Felleisen wrote:
> Yes. This is an HLI problem that we wish to tackle at some 
> point.  We have discussed it more than once.  Ideas and 
> implementation contributions welcome. -- Matthias

I just pushed a first step towards a solution. Now, only the most
general cases (ignoring filters) are shown. In the case of <, that
means that the only domain shown is: Real Real *

I also started fixing the errors caused by a misuse of + and other
arithmetic operators, by showing only the domains that are consistent
with the expected types. These messages are still not quite
satisfying, but we have a couple ideas to improve them.

Vincent