Set appropriate 500 and 404 documents for HHVM

Configure hhvm.server.error_document{404,500} for pretty error pages:
hhvm-fatal-error.php for 500s (added by MaxSem in change I9e97418aa /
ce7cc3b5ae9) and the standard 404.php for 404s.

You won't find these configuration keys if you grep the HHVM source tree.
The ini-style lowercase_with_underscore names are programmatically derived from
the hdf-style ErrorDocument404 and ErrorDocument500 directives.

Change-Id: I1aa43dbb00745e652ef04c0ee325424167fb2c25