Allow CallbackExceptionHandler to be set on DeferredManager and Promise

[aalmiray]

The result of a quick spike to figure out if it's possible to define CallbackExceptionHandler on both DeferredManager and Promise in order to have localized exception handling. The current rules are:

• setting CEH on a Promise (once submitted) takes precedence over the other CEH (as long as it's set before any callback resolves).
• every Promise passes on its CEH to any Promise created by it, such as FilteredPromise and PipedPromise, in other words children promises inherit the CEH from their parent.
• setting CEH on DeferredManager sets it on every Promise created by this DM.

Precedence of invocation for CEH is thus set as: promise, dm, global.
Please have a look at FailureTest to see how this mechanism works.

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This change is 

[coveralls]

Coverage increased (+1.0%) to 55.717% when pulling 9c8e08e on aalmiray:local-exception-handler into 395e988 on jdeferred:2.x.

[saturnism]

My first thought at this is, what if the promise is shared w/ multiple consumers? Each consumer can attach their done/fail callbacks. But when it comes to callback exception handling, they could overwrite each other.

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Review status: 0 of 20 files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[aalmiray]

indeed. There are other problems such as: let's assume you have a Promise created by dm1, combine it with other promises from dm2 -> the first promise now has it's CEH set by dm2 (if non-null, I missed that check). This definitely requires more work, specially coming up with possible scenarios

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Review status: 0 of 20 files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable