vim: use try catch in vim plugin to avoid conflicts

bundle2: raise BundleValueError error for stream level unsupported params This ensures both consistency and smooth propagation over the wire.

bundle2: support None parttype in BundleValueError This will be used for errors at the stream level.

bundle2: ignore advisory part with unknown parameters Advisory parts are advisory. If a handler exists but does not support the proper parameters, we can safely ignore it. Test has been updated to include this case.

bundle2: enforce all parameters in a part to be handled Once we picked a handler, we check that all mandatory parameter keys are properly supported. If not we raise an exception. We added a test for this case. The code now fails for any part with unknown mandatory parameters. We will ignore such errors for advisory parts in a later changeset.

bundle2: declare supported parameters for all handlers We now update all existing handlers with the supported parameters information.

bundle2: make it possible to declare params handled by a part handler If we are to enforce the mandatory aspect of parameter, we need a way to discover what a handler supports. The best option we end up with is this a simple declaration of known parameters at registration time. We simply plug the list of parameters on the function object because Python lets us do that and there is no benefit for a more complicated way. One of the handlers is updated for example and testing.

bundle2: support transmission of params error over the wire We picked a null character to split each parameter during the transfer. This is fragile if the same character is used in parameter name. However other codes will already behave in a strange way in that case, so we are not introducing any regression. A better format may be picked for the final version of the protocol.

bundle2: introduce a ``params`` attribute to BundleValueError We'll first use it for unsupported mandatory parameters on parts.

bundle2: introduce a parttype attribute to BundleValueError We will use the Exception for more that just unknown part type.