Continued improvement to exception handling code (issue207059)

[johnfargo]

Reviewers: shindig.remailer_gmail.com,

Description:
I'm committing this code early since we need it in fairly short order
and it seems quite low-risk: familiar design pattern, no semantic
difference in any behavior, just cleanup. Still, the line count is
somewhat higher than I'm comfortable committing blindly, so I'm putting
this out for review. Happy to clean up per anyone's comments as a
follow-up.
Please review this at http://codereview.appspot.com/207059/show

Affected files:
   
java/gadgets/src/main/java/org/apache/shindig/gadgets/AbstractSpecFactory.java
  java/gadgets/src/main/java/org/apache/shindig/gadgets/GadgetException.java
   
java/gadgets/src/main/java/org/apache/shindig/gadgets/process/Processor.java
   
java/gadgets/src/main/java/org/apache/shindig/gadgets/render/DefaultRpcServiceLookup.java
   
java/gadgets/src/main/java/org/apache/shindig/gadgets/render/HtmlRenderer.java
   
java/gadgets/src/main/java/org/apache/shindig/gadgets/render/OpenSocialI18NGadgetRewriter.java
  java/gadgets/src/main/java/org/apache/shindig/gadgets/render/Renderer.java
   
java/gadgets/src/main/java/org/apache/shindig/gadgets/render/RenderingException.java
   
java/gadgets/src/main/java/org/apache/shindig/gadgets/render/RenderingGadgetRewriter.java
   
java/gadgets/src/main/java/org/apache/shindig/gadgets/rewrite/CssRequestRewriter.java
   
java/gadgets/src/main/java/org/apache/shindig/gadgets/rewrite/HTMLContentRewriter.java
   
java/gadgets/src/main/java/org/apache/shindig/gadgets/rewrite/RewritingException.java
   
java/gadgets/src/main/java/org/apache/shindig/gadgets/rewrite/TemplateRewriter.java
   
java/gadgets/src/test/java/org/apache/shindig/gadgets/rewrite/CssRequestRewriterTest.java
   
java/gadgets/src/test/java/org/apache/shindig/gadgets/rewrite/HTMLContentRewriterTest.java