Fix mvn gwt:test

Fix mvn gwt:test

	* pom.xml: Use htmlunit mode for gwt:test, because the default demands
	user-interaction, asking us to load a temporary URL in a browser.s
	* src/main/java/org/glom/web/OnlineGlom.gwt.xml: Add a servlet node,
	which is apparently necessary for testing the service. See the comment.
	* src/test/java/org/glom/web/client/GwtTestOnlineGlomService.java:
	Show the exception, if any. This is how I saw the 404 in the HTML in
	the exception.