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.

bundle2: rename b2x:error:unknownpart to b2x:error:unsupportedcontent This is a backward compatibility breakage per se. But bundle2 was explicitly flagged as experimental, and this is one an error path anyway. So the worse possible outcome from this change is to still have a crash but with a different message.

bundle2: move exception classes into the error module Exceptions should have known their place.

bundle2: rename UnknownPartError to BundleValueError We are going to raise exceptions for a wider range of cases: unsupported mandatory stream and part parameters. We rename the exception with a wider name.