Syntax of rules should be validated

[stianstr]

As rules are Python modules that are imported by reactor, syntax error in a rule causes an exception when reactor attempts to import it. Reactor does not handle errors during these imports well, and to make things worse it wipes all existing rules before attempting import. Meaning that if one import rules one is left with a non-functioning reactor. One then needs to fix the error and restart reactor.

We should ensure rules are validated before attempting to import them, and discard any invalid rules with a warning rather than a crash.