Cannot raise UnsupportedMethod in DeferredResource

New submission from 107269088 <None>:

This is completely lame. Due to the way that exceptions are handled in a deferred
and the fact that server.site.render relies on receiving the UnsupportedMethod exception
for flow control, makes it impossible to really effectively use the DeferredResource.

Using exceptions for flow control may be pythonic, but it's wrong in this case. Arguably,
UnsupportedMethod isn't all that exceptional considering how much processing it does. Better
would be to have render return a special error code if it can't find a method for the specified
HTTP METHOD instead of raising an exception.

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Type     : defect
Component: web
Keywords : 
Priority : highest
Nosy     : 
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http://twistedmatrix.com/trac/ticket/3906