sdbus++: generate error cpp

Change-Id: I63ea5ea07ab4efb0da21ee10de7fe9db429c1ffd
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/tools/sdbus++ b/tools/sdbus++
index db96b1f..28c2858 100755
--- a/tools/sdbus++
+++ b/tools/sdbus++
@@ -14,7 +14,8 @@
     valid_processes = {'markdown': "markdown",
                        'server-header': "server_header",
                        'server-cpp': "server_cpp",
-                       'exception-header': "exception_header"}
+                       'exception-header': "exception_header",
+                       'exception-cpp': "exception_cpp"}
 
     parser = argparse.ArgumentParser(description='Process sdbus++ YAML files.')