error: store filename and message on LookupError for later It may be useful to be able to get to the failed path and the underlying error message when catching a LookupError, so let's make them accessible.

bundle2.unbundlepart: implement seek() This implements a seek() method for unbundlepart. This allows on-disk bundle2 parts to behave enough like files for bundlerepo to handle them. A future patch will add support for bundlerepo to read the bundle2 files that are written when the experimental.strip-bundle2-version config option is used.

bundle2.unbundlepart: tell() implementation Keep track of how many bytes we've returned from read(); fairly straightforward.