xdrgen: improve error reporting for invalid void declarations

xdrgen: improve error reporting for invalid void declarations

RFC 4506 defines void as a zero-length type that may appear only as
union arms or as program argument/result types. It cannot be declared
with an identifier, so constructs like "typedef void temp;" are not
valid XDR.

Previously, xdrgen raised a NotImplementedError when it encountered a
void declaration in a typedef. Which was misleading, as the problem is an
invalid RPC specification rather than missing functionality in xdrgen.

This patch replaces the NotImplementedError for _XdrVoid in typedef
handling with a clearer ValueError that specifies incorrect use of void
in the XDR input, making it clear that the issue lies in the RPC
specification being parsed.

Signed-off-by: Khushal Chitturi <kc9282016@gmail.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>