rxgen: Emit individual exception classes for defined abort codes and raise them

rxgen: Emit individual exception classes for defined abort codes and raise them

Emit an exception class for each defined abort code and raise the appropriate
exception when an abort is received.  Each set of classes is subclassed from a
package-specific class (eg. AbortVL_NOENT -> VLAbort) and each
package-specific class is subclassed from a generic class (eg. VLAbort ->
RemoteAbort) so that all package codes can be trapped in one statement and all
remote aborts can be trapped together.

If an unknown code is received, then this is raised as RemoteAbort only.

RxRPC/rxgen codes are supplied now also in the rxgen.xg file under the RXGEN
package name.

KAA/KAM/KAT codes are under the KA package.

Signed-off-by: David Howells <dhowells@redhat.com>