Add a --verbose-failures option. Write "checked n/n" to stderr
Treat symlinks correctly

Previously, the hash value for a symlink was of the contents of its target.
Now, hash the path that the link points to. As a consequence, broken symlinks
no longer cause this script to fail.
Fix unused verbose_failures argument
Fix HashEntry.exists to handle symlinks

Not using os.path.exists: this should return False for directories and True
for broken symlinks.
Fix updating database version in load()