Use generator to generate exceptions from yaml

Signed-off-by: Asai Neko <sugar@sne.moe>

cmd/gen_exception/exception.tmpl

@@ -0,0 +1,8 @@
+// Code generated by gen-exception; DO NOT EDIT.
+package exception
+
+const (
+{{- range .Items }}
+	{{ .Name }} = "{{ .Value }}"
+{{- end }}
+)