readv(directory, ...) fails with AttributeError; should give more meaningful error

[Michael Hudson-Doyle]

I don't think this is what was intended:

>>> from bzrlib.transport import get_transport
>>> get_transport('.').readv('.', [(0, 1)]).next()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.6/dist-packages/bzrlib/transport/__init__.py", line 697, in _seek_and_read
    fp.seek(c_offset.start)
AttributeError: 'LateReadError' object has no attribute 'seek'

I guess LateReadError should implement seek().