scan(): If the status byte has an unexpected value, show it in hex

rather than as a (probably) unprintable character.

src/ZEO/cache.py

@@ -661,7 +661,7 @@ class FileCache(object):
             elif status in '1234':
                 size = int(status)
             else:
-                assert 0, status
+                assert 0, hex(ord(status))
 
             self.filemap[ofs] = size, ent
             if ent is None and size > max_free_size: