Set default value for error message

This seems to be hard to reproduce as an error, but happened
following the logs.

Change-Id: I0b33996ff77efd8b3cb859270e1c9d677c73b0ef
diff --git a/src/main/resources/assets/templates/export.ftl b/src/main/resources/assets/templates/export.ftl
index 947b7ba..71d529c 100644
--- a/src/main/resources/assets/templates/export.ftl
+++ b/src/main/resources/assets/templates/export.ftl
@@ -122,7 +122,7 @@
     <#if code??>
     <script>//<![CDATA[
       function dynCall (P) {
-        P.log(${code}, '${msg}');
+        P.log(${code}, '${msg!"Error!"}');
       };
     //]]></script>
     </#if>