dg-excess-errors is a very large hammer that is rarely what you want to 
use.  To filter out certain messages, it is better to use 
dg-prune-output.  So this patch removes most of the dg-excess-errors 
from the v3 testsuite, replacing some with dg-prune-output, but most 
need no replacement.

Doing this revealed that the bind/ref_neg.cc test was broken; the 
dg-error tags were missing a space before the closing }, so they were 
being ignored.  When I fix that, I find that two of the tests are failing.

I'm committing the first patch, but am holding off on the second for 
now.  Should I go ahead and commit it or would someone like to fix those 
bugs and then commit it themselves?