BLS pairing raise on failure

[arvidn]

These are some of the important points:

• any operand in clvm is evaluated eagerly, so operations like these can easily be combined using any of multi-arg operator. But perhaps the most convenient would be the function proposed for the standard library (filtermap (bls_verify ...) (list conditions...))
• raising on failure eliminates a class of errors (it's debatable how important it is though)
• raising on failure guides people towards setting up their puzzles and solutions such that you always know that a verification will pass, or avoid running it at all, which I believe is best practice.
• raising on failure (and returning nil) is compatible with unknown operators, and allows future verification operators to follow the same pattern and be soft-forked in outside of the softfork-guard. secp256 is a likely future candidate.

[coveralls-official]

Pull Request Test Coverage Report for Build 5072478901

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.007%) to 91.82%

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Totals
Change from base Build 5071900008:	0.007%
Covered Lines:	4142
Relevant Lines:	4511

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💛 - Coveralls