Delete bundle file of hg incoming in case of errors, preserve existing files.
Let write_bundle always create the bundle file, check if target doesn't exist
and delete it in case of errors.
This way incoming only has to delete the bundle file if it was meant to
be a temporary file.
#!/usr/bin/env python
#
# An example CGI script to use hgweb, edit as necessary
import cgitb, os, sys
cgitb.enable()
# sys.path.insert(0, "/path/to/python/lib") # if not a system-wide install
from mercurial import hgweb
h = hgweb.hgweb("/path/to/repo", "repository name")
h.run()