pock: Report `duplicate label' as an expected error.

It shouldn't result in a backtrace.

diff --git a/pock b/pock
index 053cb36..3d9f66b 100644 (file)
--- a/pock
+++ b/pock
@@ -662,7 +662,7 @@ class PrimeProof (object):
 
     ## If the label is already taken then we have a problem.
     if step.label in me._steps:
-      raise ValueError('duplicate label `%s\'' % step.label)
+      raise ExpectedError('duplicate label `%s\'' % step.label)
 
     ## Store the proof step.
     me._pmap[step.p] = step.label