codegen: Exit with error on error generating C structures

This was some left-over during development of C structure
generations (the single structure generation was skipped).

Signed-off-by: Frediano Ziglio <fziglio@redhat.com>
Acked-by: Uri Lublin <uril@redhat.com>

spice_codegen.py

@@ -283,6 +283,7 @@ current:
         print >> sys.stderr, 'type %s' % t
         print >> sys.stderr, writer.getvalue()
         traceback.print_exc(sys.stderr)
+        sys.exit(1)
 
 def generate_declarations():
     writer = codegen.CodeWriter()