[Trinidad] PPR hides relevant exception

[my...@eekboom.com]

[Trinidad 1.0.3]
If an exception occurs during PPR, most of the time the root cause isn't 
visible in the server log. 

I used to disable PPR for debugging.
Now that I took the time to look into it, it seems that the culprit is
   XmlHttpConfigurator.handleError(ExternalContext, Throwable)
It _LOG's the caught throwable, but often that is just a servlet exception
which for some stupid reason does not yet use the standard nested exception
mechanism.
So all I get in the log is the servlet exception but not the real reason, 
say a NullPointerException.

Just a couple of line below (where the exception string for the client side
message is retrieved), there already is code to unwrap "uninteresting" 
exceptions.

I propose to make use of that for the logging, too.