Handle decryption of ACS results failing

[Vagabond]

If the transaction bundle produced by an ACS instance results in a
failure to decrypt, return the empty list as the result of that ACS and
continue to checking if enough ACSes have completed and we can return a
list of transactions to the user.

[Vagabond]

I'm going to add a unit test for this, don't merge yet.

[coveralls]

Pull Request Test Coverage Report for Build 353

• 15 of 17 (88.24%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+1.2%) to 83.843%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/hbbft.erl	15	17	88.24%

Files with Coverage Reduction	New Missed Lines	%
src/hbbft_bba.erl	1	83.11%

Totals
Change from base Build 328:	1.2%
Covered Lines:	384
Relevant Lines:	458

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💛 - Coveralls

[evanmcc]

looks good. is the meat of the change that we now count them as failed instead of continuing to anticipate that they might show up somehow?

[Vagabond]

Yes, essentially. I think, however, we should wait for 2f+1 shares before we declare it unsalvageable (because we might have gotten 1 invalid share but f invalid shares). I'm pushing a change to address this.