Created special exception hierarchy for Prover9 errors.  Fatal errors indicate serious issues such as malformed syntax.  LimitExeeded exceptions indicate that Prover9 has cut itself off because the proof was taking too long.  The distinction is useful because the undecidablity of first-order logic means that it may be useful to simply assume that long-running proofs are non-theorems.
Created special exception hierarchy for Prover9 errors.  Fatal errors indicate serious issues such as malformed syntax.  LimitExeeded exceptions indicate that Prover9 has cut itself off because the proof was taking too long.  The distinction is useful because the undecidablity of first-order logic means that it may be useful to simply assume that long-running proofs are non-theorems.

svn/trunk@8647

nltk/nltk,nltk/nltk,nltk/nltk