Bug 541: Failure to load an external script is not fatal

http://code.google.com/p/google-caja/issues/detail?id=541
    if <script src="..."> doesn't NPE ( issue 540 )
    and the external js can't be loaded,
    caja emits PluginMessageType.FAILED_TO_LOAD_EXTERNAL_URL,
    which is a warning.

Changed RewriteHtmlStage to throw an exception instead so that a
user installed error handler can deal with it.


Submitted @3762

[felix8a, 2009-09-27 20:17:04]

lgtm