Check error handlers for specific classes first

This allows adding error handlers like this:

    @app.errorhandler(werkzeug.exceptions.Forbidden)

And subclassing HTTPExceptions:

    class ForbiddenBecauseReason(Forbidden): pass

    @app.errorhandler(ForbiddenBecauseReason)
    def error1(): return "Forbidden because reason", 403
    @app.errorhandler(403)
    def error2(): return "Forbidden", 403

... the idea being, that a flask extension might want to raise an
exception, with the default behaviour of creating a HTTP error page,
but still allowing the user to add a view/handler specific to that
exception (e.g., "Forbidden because you are not in the right group").

CHANGES

@@ -14,6 +14,12 @@ Version 1.0
   `False` it will only be modified if the session actually modifies.
   Non permanent sessions are not affected by this and will always
   expire if the browser window closes.
+- Error handlers that match specific classes are now checked first,
+  thereby allowing catching exceptions that are subclasses of HTTP
+  exceptions (in ``werkzeug.execptions``).  This makes it possible
+  for an extension author to create exceptions that will by default
+  result in the HTTP error of their choosing, but may be caught with
+  a custom error handler if desired.
 
 Version 0.10.2
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