pure: use string for another exception in the pure version of base85 stable

pure: use string for another exception in the pure version of base85

That message does not seems tested, but I am assuming that the same reasoning as
for the previous changeset applies.

--- a/mercurial/pure/base85.py	Sat Nov 16 20:07:49 2019 +0100
+++ b/mercurial/pure/base85.py	Sat Nov 16 20:08:35 2019 +0100
@@ -70,7 +70,7 @@
                     'bad base85 character at position %d' % (i + j)
                 )
         if acc > 4294967295:
-            raise ValueError(b'Base85 overflow in hunk starting at byte %d' % i)
+            raise ValueError('Base85 overflow in hunk starting at byte %d' % i)
         out.append(acc)
 
     # Pad final chunk if necessary