Include deferred callback history in the formatted rendering of Failures

[twisted-trac]

	@radix reported
Trac ID	trac#6589
Type	enhancement
Created	2013-06-21 21:30:13Z

#6538 adds a facility to Deferred to track the history of callbacks. This history should be given to related Failures and when a Failure's traceback is formatted it should display the history in an easy-to-read way.

To be a bit more concrete about the expected behavior:

• if an exception is raised from a callback, the Deferred's history should be included in the automatically created Deferred
• if a Failure is returned from a callback, the Deferred's history should be inserted into that Failure (assuming it doesn't already have one?)

Thus the Deferred history as included in a Failure represents the history of the Deferred at the point at which there was an error (the history need not implicitly update with further callbacks after this event; if it does, it should be very explicitly demarcated as post-error in the output).

Then, when the Failure is formatted through its various formatting methods, the Deferred history should be rendered along with the traceback.

Searchable metadata

``` trac-id__6589 6589 type__enhancement enhancement reporter__radix radix priority__normal normal milestone__ branch__ branch_author__ status__assigned assigned resolution__None None component__core core keywords__ time__1371850213000000 1371850213000000 changetime__1373000683000000 1373000683000000 version__None None owner__radix radix ```

[twisted-trac]

	@radix set owner to @radix @radix set status to assigned