ExternalResource: declare after() to throw Throwable (junit-team#1421)

This allows clients to call methods that throw checked exceptions without having to catch and wrap checked exceptions.

src/main/java/org/junit/rules/ExternalResource.java

@@ -77,8 +77,10 @@ protected void before() throws Throwable {
 
     /**
      * Override to tear down your specific external resource.
+     *
+     * @throws Throwable if teardown fails
      */
-    protected void after() {
+    protected void after() throws Throwable {
         // do nothing
     }
 }