Clean up error handling for proxy code

[awiner]

Reviewers: shindig.remailer_gmail.com,

Message:
A few small changes:
- move error handling out of "proxy" servlets and into the handlers
- handle invalid refresh params
- issue WARNING, not FINE or INFO if a GadgetException comes from an
internal server error (and send a 500, not a 400)



Please review this at http://codereview.appspot.com/63218

Affected files:
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java/gadgets/src/main/java/org/apache/shindig/gadgets/servlet/ConcatProxyServlet.java
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java/gadgets/src/main/java/org/apache/shindig/gadgets/servlet/MakeRequestHandler.java
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java/gadgets/src/main/java/org/apache/shindig/gadgets/servlet/MakeRequestServlet.java
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java/gadgets/src/main/java/org/apache/shindig/gadgets/servlet/ProxyBase.java
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java/gadgets/src/main/java/org/apache/shindig/gadgets/servlet/ProxyHandler.java
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java/gadgets/src/main/java/org/apache/shindig/gadgets/servlet/ProxyServlet.java
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java/gadgets/src/test/java/org/apache/shindig/gadgets/servlet/MakeRequestHandlerTest.java
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java/gadgets/src/test/java/org/apache/shindig/gadgets/servlet/ProxyBaseTest.java
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java/gadgets/src/test/java/org/apache/shindig/gadgets/servlet/ProxyHandlerTest.java