Minor README updates.
A possibly major improvement to the error analysis.

It can now use /positive/ information in its inferences.  Unfortunately
this introduces a small bug where the generalizer does not get called
at all, so that will have to be fixed soon.
More accurate "success code" removal

Before, the code was trying to remove an exact error descriptor from
the currently recognized error descriptors for a function.  In some
cases, the relevant error code had a different representation (with
a different witness).  The new code removes descriptors based on just
their error code (if necessary, removing one error code from a larger
set).
Remove some debug code
Major bugfixes for the error analysis

1) The code to remove success codes from the error codes list
   was short-circuiting further analysis passes, which caused
   some errors to be missed

2) The known error pass was not populating the list of known
   error codes (which meant that the list could not be learned
   from).
Add a few more tests for the error analysis
Documentation on the new reportsSuccess pass