fix prev; offset is relative to last forking height

lib/blockchain.py

@@ -226,9 +226,6 @@ class Blockchain(util.PrintError):
                 if truncate and offset != self._size*80:
                     f.seek(offset)
                     f.truncate()
-                    self.print_error(
-                        'write. truncating to offset {}, which is around chunk {}'
-                        .format(offset, offset//80//2016))
                 f.seek(offset)
                 f.write(data)
                 f.flush()