Implement error transformer for resultToPromise

[seprich]

Previous change to the resultToPromise function throws the contents of errors away replacing it with meaningless FutureError. This PR fixes this bug, but is not backwards compatible because there is a new parameter errorTransformer.

[coveralls]

Pull Request Test Coverage Report for Build 78

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.03%) to 98.675%

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Totals
Change from base Build 76:	-0.03%
Covered Lines:	132
Relevant Lines:	133

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💛 - Coveralls

[scull7]

I'm also on-board with these changes

[briangorman]

@seprich Could you implement this as an optional parameter with a default transformer keeping the current future error as the default? Ultimately I don't think many people are clamoring for this feature and the current behavior is "good enough" for most use cases, but I can understand some users wanting greater control

[seprich]

In fromPromise the errorTransformer is not an optional parameter, why should resultToPromise be any different? Also how is ignoring errors "good enough"? Before the recent fix of test library the tests may not have worked as expected and someone may have assumed that this function (resultToPromise) worked better than it did but in fact it just throws the error contents away. I'd consider that as a bug, not a feature. You are free to disagree of course.