Unpack error handler when registering on middleware

[RobbeSneyders]

Registering error handlers on the async app leads to an error on 3.0.0a5 because the error handlers are not unpacked correctly when being registered internally.

[coveralls]

Pull Request Test Coverage Report for Build 4787198260

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 93.841%

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Totals
Change from base Build 4774497315:	0.2%
Covered Lines:	3337
Relevant Lines:	3556

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💛 - Coveralls

[Ruwann]

LGTM, thanks for adding a test!