[regtool] Fail properly if something goes wrong

Signed-off-by: Rupert Swarbrick <rswarbrick@lowrisc.org>
diff --git a/util/reggen/gen_html.py b/util/reggen/gen_html.py
index 5515255..0a53f1d 100644
--- a/util/reggen/gen_html.py
+++ b/util/reggen/gen_html.py
@@ -296,4 +296,4 @@
                             toclist, toclevel)
             continue
 
-    return
+    return 0