Support for special treatment of errors

I wanted to log errors. So I patched it in here. Some providers (i.e. Flurry) has support for error logging via `+ (void)logError:(NSString *)errorID message:(NSString *)message error:(NSError *)error;`

I'd love to hear what's your opinion on that. The default implementation in `ARAnalyticalProvider.m` is a bit hacky, though.