Symlink check breaks on Windows

[Lee Cantey]

The following changeset gets an AttributeError on windows when trying
to call os.symlink.  I haven't looked at the usage but catching the
AttributeError gets it working again.

changeset:   3998:315d47991fd4
user:        Matt Mackall <mpm at selenic.com>
date:        Fri Dec 29 20:04:31 2006 -0600
summary:     symlinks: check whether a filesystem supports symlinks