Added urlpattern exception handling

Author: pmeenan

fetch.bs

@@ -6657,7 +6657,7 @@ run these steps:
    <li><p>Let <var>pattern</var> be the result of
    <a for=/>creating a URL pattern</a> given the <a>bare item</a> of <var>dictionaryValue</var>["<code>match</code>"],
    the <a lt="URL serializer">serialization</a> of <var>request</var>'s <a for=request>current URL</a>,
-   and an empty map.
+   and an empty map. If this throws an exception, then return <var>response</var>.
 
    <li><p>If <var>pattern</var> is failure or <var>pattern</var> <a for=/>has regexp groups</a>,
    then return <var>response</var>.