breaking change: don't return Result<> from bam::Record::push_aux

[pmarks]

As discussed in #147. Applies on top of #148.

[coveralls]

Pull Request Test Coverage Report for Build 653

• 12 of 14 (85.71%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.02%) to 60.146%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/bam/record.rs	8	10	80.0%

Files with Coverage Reduction	New Missed Lines	%
src/bam/buffer.rs	1	55.29%

Totals
Change from base Build 647:	-0.02%
Covered Lines:	21471
Relevant Lines:	35698

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💛 - Coveralls

[pmarks]

@johanneskoester anything else needed here?

[johanneskoester]

Perfect, thank you!