Add unittest.nse to 'safe' category
Define in-use-but-undefined ScriptDBSyntaxError

Subclassed SyntaxError to provide some useful info when this happens. It
was happening with unittest.nse because it wasn't part of any category.
Previously, this would crash Zenmap because ScriptDBSyntaxError was
undefined. Now it crashes because there's really a syntax error (fixed
in previous revision)