[Seaside-dev] error handling changes

[Julian Fitzell]

I've been cleaning up error handling today. I wanted to just post a
list of changes (taken directly from the commit messages) so that
people will see it and point out anything really stupid I did and also
so if we find problems in the next few weeks we'll remember why it was
changed the way it is.

Here it is:

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Clean up error handling:

+ Remove WADefaultErrorHandler and WASimpleErrorHandler
+ Make WAErrorHandler always output a very simple text/plain error
message as a failsafe
+ Add #handleDefault: to WAErrorHandler and have both #handleError:
and #handleWarning: call it so behaviour can be easily specified for
both of them in one location
+ remove WAReponse class>>internalError:'s dependency on WACanvas
+ Make sure that all error handlers ALWAYS ALWAYS ALWAYS return a response
+ Delete WAPageExpired since it was poorly named for what it did and
was only used when a user tried to proceed on a non-resumable
exception. This case is now handled by returning an internal error
response.
+ Make WAWalkbackErrorHandler a subclass of WADebugErrorHandler

Remaining issue is that when you close the debugger, a nil response is
returned. WAKom handles this by displaying a message saying the page
is expired but this would be better handled by catching somewhere and
then returning a response with an explanation.
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