referencesf(): Use %r instead of %s format in the error message, else

the pickle shown is full of unprintable characters.

NEWS.txt

@@ -39,6 +39,13 @@ _Persistence.c
   has its own copy, and the C code hadn't been compiled in ZODB since before
   ZODB 3.3.
 
+Error reporting
+---------------
+
+- In the unlikely event that ``referencesf()`` reports an unpickling error
+  (for example, a corrupt database can cause this), the message it
+  produces no longer contains unprintable characters.
+
 
 What's new in ZODB3 3.4b1?
 ==========================

src/ZODB/serialize.py

@@ -514,7 +514,7 @@ def referencesf(p, rootl=None):
         u.persistent_load = []
         u.noload()
         if len(p) > f.tell():
-            raise ValueError, 'Error unpickling, %s' % p
+            raise ValueError, 'Error unpickling %r' % p
 
 
     # References may be: